Mechanical Engineering Theses and Dissertations

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Boiling Heat Transfer to Liquid Nitrogen and to Liquid Helium
(1960) Mann, Horace Tharp; Shreeve, C.A. Jr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
Experimental data on free convection and nucleate boiling was taken in liquid nitrogen and liquid helium using platinum wires as heating elements. The results in liquid helium and particularly in liquid nitrogen were found not to agree with the generally accepted results in other liquids. In particular it was found that the transition from free convection to nucleate boiling would not take place until the temperature of the wire was much greater than that found for nucleate boiling. An "extended region" thus must be added to the free convection curve. This extended region did not reoccur in the reverse transition from nucleate boiling to free convection. It is usual to represent nucleate boiling heat transfer data to liquids as an equation of the form g/a = CΔT^n where g/ a is the rate of heat transferred per unit area, ΔT is the excess temperature of the heating surface and C and n are independent constants. There is universal agreement that 2.5 ≤ n ≤ 4 for all liquids. However, it is found for liquid nitrogen that n is not in this region but is about 11. It is then shown that C is a function of n and the equation in liquid nitrogen reduces to one with a single arbitrary constant of the form. g/a = exp^(10.25 - 2.45n) ΔT^n 5 ≤ n ≤ ∞ This equation represents a family of curves which intersect at the maximum observed value of g/a for . 0,008 inch wires. The existing mechanisms used to explain the high heat, transfer rates in nucleate boiling are reviewed and shown to be quantitatively invalid in liquid nitrogen. A "hot" molecule hypothesis is proposed, wherein a single hot molecule is assumed to supply all of the energy requirements for the growth of a bubble. It is shown that this hypothesis is invalid in itself but the calculations lead to an alternative hypothesis. This alternative hypothesis proposes that the excess energy stored in the bubble boundary acts as an energy sorting mechanism which must be present to keep a growing bubble from violating the laws of' thermodynamics. Experimental data is presented tor nucleate boiling from platinum wires in liquid helium. This data is also unusual but is more or less consistent with the results obtained in liquid nitrogen. An extended region is not, however, observed in liquid helium.
Gamma Ray Spectra and Shielding Survey of the University of Maryland Reactor
(1963) Nhiep, Nguyen; Duffey, Dick; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
The gamma ray spectra of the University of Maryland reactor were measured at the beam tube which extends from the side of the reactor to the core, the thermal column and the top of the reactor. There were three gamma ray spectra measurements at the beam tube : gamma ray spectra recorded by a single-channel spectrometer when the reactor was shut down and when the reactor was operated at different power levels and the gamma ray spectra recorded by a 256-channel spectrometer when the reactor was at different levels of power. However, only the single channel spectrometer was used to measure gamma ray spectra at the thermal column and the top of the reactor; some gamma ray spectra were recorded at the thermal column when the reactor was operated at various power levels and a gamma ray spectrum was recorded at the top of the reactor when the reactor was operated at full licensed power of 10 kw. The gamma ray shielding survey of the reactor was done by a Geiger-Muller survey meter when the reactor was running at full power of 10 kw. The highest level found was about 0.8 millirem per hour which was at the side of the reactor at core level. A calculated gamma ray level was near this value.
Cerenkov Light Production in a Water Moderated Nuclear Reactor
(1963) Madey, Robert William; Duffey, Dick; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
An experimental investigation of the production of Cerenkov radiation in a water moderated nuclear reactor is conducted using a photomultiplier as a light sensor. The variations in light intensity are studied during various phases of reactor operation, namely: startup, steady state and shutdown. The relevant theory is presented as an aid in interpreting and extrapolating the experimental results. It is found that for transients such as startup, the light signal is directly related to reactor power for periods (e - folding time) faster than about 20 seconds. Additional transient data acquired from measurements performed on a TRIGA pulsed-type reactor illustrate the excellent agreement between the Cerenkov detector and a conventional ionization chamber for measuring pulse characteristics such as peak power, pulse half-width, and prompt period. The proportionality between reactor power and Cerenkov signal is no longer valid for whole core measurements made at steady state power level because of the gradual increase of the Cerenkov signal as a result mainly of fission product contributions. Selective scanning of the Cerenkov spectrum through the use of interference filters over the wavelength range 3500 Å to 5530 Å results in a lower buildup fraction. Indications are that measurements further into the short wavelength region may yield a light sensor, and hence a good power detector, independent of any fission product buildup. The decrease in the Cerenkov light intensity after shutdown is measured for reactor operating times from 20 minutes to 4 hours. Comparison of the empirical data with theoretical considerations results in good agreement for shutdown times ranging from 500 seconds to 10,000 seconds. Spectral measurements made through 17 feet of water with a Hilger quartz spectrograph show a spectral distribution ranging from 2500 Å to 6000 Å. A calculated spectral distribution is compared with the measured spectrum after correcting for water attenuation.
A Study of Combustion in Supersonic Streams
(1964) Billig, Frederick Stucky; Shreeve, Charles A. Jr.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
Supersonic combustion of reactive aluminum alkyl fuels has been experimentally demonstrated in two-dimensional ducted combustors and adjacent to a flat plate. Fuel was injected from the combustor walls through multiple orifices and ignited spontaneously. Stable supersonic heat release was maintained as evidenced by schlieren and direct motion pictures of the flow field and deduced from static and pitot pressure measurements in the combustion zone. The results of the ducted combustor tests were correlated with elementary one-dimensional and pseudo-one-dimensional theoretical models of the flow field. This agreement permitted a reason.able determination of combustion efficiency to be made. In the ducted combustor tests a favorable effect of preheating the fuel to approximately 250°F was noted and a simple empirical factor was found which satisfactorily correlated all of the data for the range of conditions tested. A theoretical model of constant pressure heat release on a flat plate in supersonic flow is postulated. Normal force coefficients and specific impulse values are tabulated for a variety of flight Mach numbers and altitudes. Additional refinements in this theoretical model were required to adequately describe the experimental results. In a test simulating Mach 5 flight at 66,000 feet altitude a side force specific impulse of 1350 seconds was measured at equivalence ratio of one. Combustion was only partially completed 12 inches downstream of fuel injection. Based on the theoretical mode l an additional 12 inches of combustor length and 36 inches of expansion length would be required to obtain the estimated theoretical impulse of 5760 sec. The interaction of a vaporizing liquid droplet with a supersonic stream is considered. Additional refinements were made in the existing theories on droplet trajectory to include the influences of a separated zone and the normal component of velocity of the external stream. Calculations of the trajectory and evaporation of the estimated mean droplet size based on the modified technique were in general agreement with the observed flame zone and deduced combustion efficiency.
Fast Neutron Flux in the University of Maryland Reactor
(1965) Berman, Philip Gilbert; Duffey, Dick; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
The fast neutron flux distribution in the experimental facilities of the University of Maryland Reactor (UMR) was studied by means of neutron activation threshold foils with the reactor at a power level of 10 kw. The foils used were sulfur, magnesium, and aluminum which detected neutrons of energies greater than 3.0 Mev, 6.3 Mev, and 8.1 Mev, respectively. The activities of the foils were measured by calibrated beta and gamma scintillation detection systems. A central row of fuel elements, the west beam port, and the through tube were studied using all three types of foils. The fast neutron flux above 3.0 Mev was measured in each element of the core. The maximum fast neutron flux above 3.0 Mev available in the central ''glory hole " was about 2x10^10 n/cm^2 -sec. The flux above 6.3 Mev and above 8.1 Mev was about 5.5x10^8 n/cm^2 -sec and about 4.5x10^8n/cm^2 -sec, respectively. Available in the west beam port was a maximum fast neutron flux above 3.0 Mev of about 2.6x10^9 n/cm^2 -sec adjacent to the core and of about 1.2x10^7 n/cm^2 -sec at the inner face of the beam port shield plug. The fast neutron flux above 3.0 Mev available in the center of the through tube was 1.7x10^9 n/cm^2 -sec. The minimum at both east and west ends, at the inner faces of the shield plugs, was about 7x10^5 n/cm^2 -sec.
The Effect of Nuclear Reactor Radiation on the Electrical and Mechanical Properties of Epoxide Polymers
(1965) Kincaid, Charles Vernon; Duffey, Dick; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
The effect of nuclear reactor radiation on the volume resistivity, bulk compressibility, specific volume, and hardness of three epoxide polymers was studied. The polymers were thermally polymerized and then subjected to varying radiation doses in the enriched uranium, water moderated, heterogeneous nuclear reactor located at the University of Maryland. Changes in the volume resistivity as a function of temperature for the various radiation doses was then determined by the use of an electrometer to measure the potential drop across the specimen. The effect of radiation on the bulk compressibility was determined by means of a special compressibility tester over the pressure range one to 8,000 atmospheres and temperatures 25°C to 250°C. The specific volume and hardness were measured for each sample before and after irradiation using standard techniques. The temperature dependence of the electrical volume resistivity was initially increased by exposure to 2.5 Mrads. Further irradiation up to 25 Mrads caused severe degradation of the temperature dependence of the electrical resistivity for all epoxide polymers studied . The magnitude of the compressibility for the polymers was found to be relatively unaffected by irradiation up to 25 Mrads; however, determination of the pressure-induced glass transition yielded anomalous results. No significant change in the hardness of the polymers was noted after varying amounts of radiation. The specific volume of an aromatic cured epoxide was not affected to any extent , but the aliphatic cured polymer showed a decreased specific volume.
Correlation Studies of Pressure Fluctuations on the Ground Beneath a Turbulent Boundary Layer
(1965) Priestley, Joseph Tant; Faller, Alan J.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
Narrow-band pressure correlation measurements in the frequency range ,008 to 1 Hz(cps) were obtained from a cross spectral analysis between pairs of microphones placed on level ground beneath the wind stream. The measurements were made over a range of wind speeds from 2.1 to 7.2 meters per second and a range of hemispheric solar radiation conditions varying from 0 to 44 Langleys per hour. Plausibility arguments are presented which predict for the narrow-band longitudinal and lateral correlation coefficients: -0'~ Rw(£,0) = e cos(kg) and Rw(O~~) = e-S~ where~ and~ are the longitudinal and lateral separations, respectively, and 0', S, and k are determined by the experiment. Contrary to similarity considerations ct and k were found not to be strictly proportional, but rather 0' = 0.4lk 1 ' 28 , 0' and k being expressed in (meters)-1 , over a range 2 < (1/ot) < 500 meters. The relation between 2 .74 Q -1 ct and S was found to be: ~ = 1. 0' , 0' and ~ expressed in (meters) , over a range 3 < (1/~) < 500 meters. For an arbitrary angle with respect to wind direction evidence is presented which indicates that Rw(~,~) is very slightly larger than the product Rw(g,O)Rw(O,~). A small amount of data taken relating the convection velocity versus wavelength to anemometer readings indicates the possibility of predicting the wind profile from pressure fluctuations on the ground.
A Computer Study of Nuclear Characteristics of the University of Maryland Reactor
(1967) Altomare, Philip M.; Duffey, Dick; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
An analytical study of the University of Maryland Reactor was performed using an IBM 7094 computer. The nuclear analysis codes employed in the study were GAM-I, for fast group cross-section evaluation, Tempest-II, for thermal cross-sections, WANDA, for one-dimensional neutron transport calculations and PDQ, for two-dimensional transport calculations. A computer program, XS-1, for the calculation of effective control rod cross-sections was written as part of this study. The method employed in the determination of fast energy broad group cross-sections for non-fuel regions from that normally used. The neutron leakage from the fuel was used as a source of neutrons in lieu of the fission spectrum. This method was felt to give a better estimate of the neutron flux distribution in energy. The analytical results compared favorably with experimental data. The predicted K- effective was 0.993 versus 1.006 actual. The thermal neutron flux calculated and measured at the center of fuel elements generally agreed within 10%. The calculated neutron flux above 3 Mev, which is more difficult to match, was found to be low by approximately 30%. Control rod worth was predicted to be 5.6% Δρ for the center shim rod and 2.7% Δρ for the outside shim rod. Measured values are reported as 3.6% Δρ and 2.0% Δρ respectively.
A Comparison of Some Analytical Models with Experiment for the University of Maryland Reactor
(1968) Ross, Malcolm Fred Jr; Duffey, Dick; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
It is often desirable in the operation of a research reactor to make adjustments in the nuclear core configuration. These adjustments may occur for a variety of reasons, such as the rearrangement of fuel to perform a particular experiment. It is beneficial to the reactor operator and experimenter to have an adequate analytical model with which to predict the changes in nuclear characteristics which occur with core rearrangement. Several analytical models have been investigated and compared with experimental results for the semipermanent, or normal, core configuration for the University of Maryland Reactor. These models were selected because, while somewhat time consuming with respect to the use of computers, the computer time utilized is much less than needed by more complex methods. At the same time, the methods used tend to minimize the large inherent error associated with simple hand calculations . The methods used consist of a two-dimensional few group diffusion theory coupled with several cross section models from which macroscopic cross sections were obtained. The cross section models used for the above thermal energy groups were the volume integrated P-1 method and the Fourier transform B-1 method. Thermal energy group cross sections we reobtained using the Wigner-Wilkins model and the Maxwell-Boltzmann model. The volume integrated P-1 model and the Wigner-Wilkins model coupled with the two-dimensional group diffusion method were found to give the best agreement with experiment for the semi-permanent core configuration. This model was then tested over a range of experiments. The conclusion of this analysis was that the model was capable of predicting, with reasonable accuracy, the changes in core reactivity with core rearrangement.
Reactivity Measurements on the University of Maryland Reactor by Conventional Methods and Statistical Processes and Comparison with Calculational Methods
(1970) Zubieta, Agustin Diaz; Duffey, Dick; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
The accurate experimental determination of nuclear reactor core physics parameters is of great importance for its safe operation. In particular, the accurate determination and prediction of criticality during the initial fuel loading of a nuclear reactor are essential for a safe nuclear reactor startup. Also, the degree of subcriticality or shutdown margin of a nuclear core with its control rods inserted is an important parameter for the operation of a nuclear reactor throughout the core operating lifetime. There are several methods utilized to determine both the criticality and the shutdown margins. All of these methods depend on measuring the response of neutron detectors and the calibration of the control rods. However, neutron detectors respond only to the neutron flux in the cor e in the vicinity of the nuclear detectors. During initial reactor core fuel loading, the reactivity of the core is determined from the multiplication of the neutrons of the startup source by the addition of nuclear fuel. Reactivity is determined from the multiplication factor by a constant which is related to the source-detector geometry in the core. In this research a method was studied which allowed the determination of reactivity independent of the source-detector geometry. Reactivity measurements of the 10 kw University of Maryland pool training reactor (UMR) were made by conventional methods and by a statistical process, the variance-to-mean ration method, and the results were compared with calculational methods. The theoretical method selected to determine the UMR core reactivity was based on the multigroup, multiregion, diffusion theory. The accuracy of the theoretical model was determined for the just critical UMR core. Agreement to within 0.2% ΔK/K was obtained between the control rod measured and the calculated reactivity for the UMR full core, and smaller UMR supercritical cores. The statistical technique of the variance-to-mean ratio of the number of counts for various counting gate openings, as a means to determine the degree of subcriticality, or shutdown margin, has been proven to be an effective method. A BF3, thermal neutron proportional detector, with a sensitivity of 12.1 counts/sec per n/cm2/sec, was placed inside of an eleven feet long aluminum tubing. The end of the tubing containing the detector was inserted in the center Glory Hole of the UMR core. The current pulses from the proportional detector was amplified and fed to a TMC 1024-channel pulse analyzer. The pulses were counted for different Δt gate openings from 10-4 seconds to 10 seconds. For each Δt gate opening, 1023 samples were taken. The printed output from the TMC-1024 was collected, giving the number of counts received per Δt, as well as the integral of all the counts received during a period of time, equal to 1023 x Δt seconds. From the integrated value for the number of counts the average count c for the gate opening Δt was obtained. The printed output was transferred to IBM cards acceptable to a "Reactor Noise" code written for the IBM-7090. This code calculated the average value, c, for each Δt (which gave a check on the validity of the data transferred to the IBM cards by comparing it with the value obtained during the measurement), the standard deviation o, and the variance-to-mean ratio for all the data taken for each Δt seconds gate opening. Plots of the values of the variance-to-mean ratio versus gate openings were obtained for several UMR full core with the rods banked at various degrees of insertion (shutdown margins), and also for various UMR subcritical cores. Measurements of the shutdown margins by the variance-to-mean technique were in agreement with the values obtained from the rod calibration for negative reactivities of less than -1. 00% ΔK/K, and within ten percent for negative reactivities of approximately -2. 0% ΔK/K. Measurements of the reactivity of small UMR cores indicated that for UMR core conditions of 0. 5% ΔK/K subcritical, experiment and theory for 1-Keff were found to be only 6 parts in 100 apart, and for 2. 0% ΔK/K subcritical, experiment and theory were found to be only 8 parts in 100 apart. The variance-to-mean technique was compared to the inverse multiplication method for determination of criticality during the UMR fuel loading, and was found to be a more accurate method, primarily, because of its independence of the source-detector geometry effects. The system utilized for the statistical data processing is exact, however cumbersome, due to the amount of data to process and the amount of peripheral hardware utilized in the reduction of the data. It appears from this study that greater overall counting efficiency for the same amount of statistical data would permit more accurate measurements at larger degrees of subcriticality, perhaps, in the region of -4. 0% ΔK / K to -5. 0% ΔK / K . A system is proposed in this study to measure negative reactivity continuously, and directly, by means of a small computer capable of accepting the output of a multichannel scaler. The computer would have a fixed internal logic capable of calculating reactivity from the variance-to-mean analysis of the neutron detector counts.
Kinetic Parameters of the University of Maryland Reactor by the Interval-Distribution Method
(1976) Simonson, Simon Christian III; Munno, Frank J.; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
The Rossi alpha of the University of Maryland Reactor was measured at criticality and at shutdown by the Babala interval-distribution method. At criticality, α= 188.8 ± 4.7 sec^-1, and at shutdown, α= 1026.6 ± 4.1 sec^-1. The shutdown reactivity was found to be ρ/β = -4.44 ± 0.14 dollars.
Expert System for Process Selection and Operation Optimization
(1986) Kar, Amit; Pandelidis, Ioannis; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
The working of an Expert System, PRODUCER, conceived as part of a CIM system for a plant manufacturing discreteparts, is presented. PRODUCER starts out by determining if the desired part can be produced. Based on part attributes it then decides on a specific manufacturing process, e.g., Welding, Casting, Forging or Machining. Having selected the process, PRODUCER establishes the particular operation. With the operation decided, PRODUCER proceeds to find all the feasible combinations, of equipment and tools, that could produce the desired component. The turning operation has been chosen to demonstrate PRODUCER's capabilities. PRODUCER then sets about the task of identifying the most optimal pair of machine-tool and cutting-tool, which will provide the highest Metal Removal Rate, MRR. This is accomplished at two levels. At the higher level, PRODUCER generates constraints, representing physical limitations of the cutting process, for each machine and tool combination. These constraints are then passed on to an Optimization program. This is a Fortran program, which operates at a lower level, and returns the optimum values of the process control variables, for each machine-tool and cutting-tool combination. PRODUCER finally yields the highest maximization of the MRR. In doing so it also identifies the particular machine-tool and cutting-tool associated with this global optimum. PRODUCER, essentially a knowledge-based production system, implemented in the First Order Predicate Logic language of Prolog, also enables intelligent adaptive control.
The Stress Field Surrounding the Tip of a Crack Propagating in a Finite Body
(1987) Chona, Ravinder; Irwin, George R.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
The goal of this dissertation was to establish the relationship between a parameter descriptive of the trajectory of a smoothly curving crack, such as the curvature of the crack path, and the local stress state in the close vicinity of the crack tip. The behavior of fast -running cracks propagating along straight and smoothly curving paths in fracture specimens of various geometries was examined using dynamic photoelasticity and representations of the running crack stress field we redeveloped in terms of the coefficients of a set of infinite series, for both opening and shear mode loading conditions. Analysis of the isochromatic patterns, using local collocation methods based on this stress field representation, allowed the stress state in the neighborhood of the propagating crack-tip to be modelled with a high degree of accuracy and results were obtained for the variations with crack tip position of both the singular and leading non- singular stress field coefficients of interest. The results obtained for quasi-static and rapid crack propagation under opening mode conditions in a ring segment revealed the importance of retaining terms of order (at a minimum) r^1/2 even when only the singular term was to be determined accurately. Furthermore, it was found that the non-singular stress field coefficients varied similarly in both static and dynamic situations, with some variations in magnitude that could be attributed to crack speed. The results from the curved crack experiments also showed systematic variation of the non-singular terms, but more importantly, it was found that the instantaneous curvature of the crack path was related to the magnitude of the lowest order non-singular stress component (the coefficient of the r^1/2 term) associated with the local shear mode of deformation in the vicinity of the tip of the running crack. Furthermore, the results established that the only singularity associated with a crack propagating along a smoothly curving path in a brittle, isotropic material was that associated with the opening mode stress intensity factor, K1, and that the shear mode singularity, KII, was identically equal to zero.
Simultaneous Measurements of the Velocity and Vorticity Vector Fields in the Turbulent Near Wake of a Circular Cylinder
(1993) Nguyen, Phuc Ngoc; Marasli, Barsam; Wallace, James M.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
Hot-wire measurements of all components of the instantaneous velocity and vorticity vectors in the wake of a circular cylinder are presented. The experiments were performed at x / d = 20 and 30, at Reα = 2000, using a miniature 12-sensor probe for the simultaneous velocity-vorticity measurements and a 4-sensor probe for the velocity-only measurements. A calibration and a data reduction method for multi-sensor hot-wire probes are introduced. The calibration technique is independent of the number of sensors and requires minimal information about probe geometry. The data reduction scheme involves the solution of an overdetermined nonlinear algebraic system of equations in a least-squares sense. The measurements indicate that the Karman vortices are accurately resolved. Statistical characteristics of the velocity aJ1d vorticity fields in this flow, including moments, probability distributions and one-dimensional spectra components are documented for the first time. Conditional analysis of the measurements at x/ d = 30 with respect, to the passage of the Karman vortices are presented. The vortex center and the saddle regions are identified and characterized. The instanteaneous velocity and vorticity signals are decomposed into mean, coherent and incoherent parts using the triple decomposition technique. A significant percentage of the incoherent fluctuations are observed to be phase-locked to the Karman vortices. The enstropy balances are computed for three decomposed parts. The generation of incoherent enstrophy due to incoherent vortex stretching is detected to be the most dominant term and is balanced by the viscous dissipation.
Heat Transfer and Pressure Drop of Liquid Cooled Offset Fin Heat Exchangers
(1993) Hu, Sen; Herold, Keith E.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
An experimental and modeling study of heat transfer and pressure drop in liquid-cooled offset fin compact heat exchangers (cold plates) is described. Liquid coolants used in the testing are water and PAO (polyalphaolefin), for which the Prandtl number ranges from 3 to 150. Attention was focused on the Reynolds number range 10 - 2000 which spans most liquid cooled applications. From the data and from comparisons with previous air-cooled data, it was found that the Prandtl number has a significant effect on the Colburn factor of the offset fin geometry but little effect on the friction factor. A numerical heat transfer analysis was performed to investigate the surface temperature distribution and uniformity of heat flux in the cold plates. The results demonstrate good agreement with surface temperature measurements. The model results were used to guide data reduction procedures. In particular, significant end effects are predicted. Through experience with the heat transfer model, these end effects were isolated. The numerical model predicts approximately uniform heat flux over the central section of the cold plates. Predictive models were developed based on a surface contribution analysis of energy and momentum balances in a unit cell of the offset fin geometry. The Prandtl number effects on heat transfer can be viewed from two perspectives: fin perspective and array perspective. The fin perspective allows explanation of the Prandtl number dependence of the periodic fully developed Nusselt number. The array perspective is analogous to the usual thermal entry region in duct flow. Thermal development from the array perspective yields higher Nusselt numbers in the entry region. The surface contribution model shows significant Prandtl number effects on offset fin heat transfer performance. The models have estimated uncertainty of ±20%. The models have been validated for heat transfer and pressure drop for Prandtl number ranging from 0.7 to 150 and Reynolds number from 10 - 2000.
Physics-of-Failure Methodology for Accelerated Thermal Cycling of LCC Solder Joints
(1995) Rothman, Timothy Paul; Dasgupta, Abhijit; Mechanical Engineering; University of Maryland (College Park, Md); Digital Repository at the University of Maryland
This research presents a case study were existing physics-of-failure models and Bayesian statistical methods are used in conjunction to quantify the test-time compression achieved during accelerated temperature cycling tests on leadless solder joints. Different combinations of substrate materials and package styles are evaluated with physics-of-failure models and calculable information is obtained from a relatively small population of test specimens under accelerated stresses, because the critical variables are identified, and their influences on the stress magnitude are quantified. Bayesian statistical analysis is employed to obtain an acceleration transform, determine the confidence on the calculations, and determine which outliers are contaminating the database. In addition to accelerating the stress levels, the total test time is further minimized by tailoring the stress drivers in each sample such that multiple stress levels can be achieved under a single loading, which eliminates the need for repeating the test at multiple load levels. This research presents the details of how the models and statistical methods are applied, the results of evaluating different material combinations and package styles, problems encountered during the test, and a summary of the acceleration transforms obtained from the test. Analytical predicative models for life predictions are essential and will obviously result in significant savings of cost and time. The methods used in this are general enough to be applied to screening, qualification, and reliability enhancement tests of a wide range of new or existing electronics assemblies.
A MANUFACTURING PROCESS AND MATERIALS DESIGN ADVISOR
(1996) Kunchithapatham, Arun; Magrab, Edward B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
A computer assisted tool, called the DESIGN ADVISOR, has been developed to help obtain a compatible set of candidate materials and manufacturing processes in a fast and straightforward manner. Information for seventeen manufacturing processes includes animations, written descriptions, still pictures and geometric design rules; information for forty-two materials includes written information and data. The DESIGN ADVISOR determines candidate manufacturing processes based on user-specified levels of one or more of seven manufacturing attributes; namely, surface condition, dimensional accuracy, complexity of shape, size, production run or production rate, and cost. The determination of candidate materials is based on the user-specified levels of one or more of eleven material attributes; namely, yield strength/density, fracture toughness/density, elastic modulus/density, high temperature strength/density, density, magnetic properties, electrical resistivity, thermal distortion, thermal insulation, solvent resistance, and cost. It also determines the suitability of candidate manufacturing processes with the candidate materials, and ranks the suitability of each candidate material within each candidate manufacturing process.
Experimental and Theoretical Investigation of Oil Retention in a Carbon Dioxide Air-Conditioning System
(2003) Lee, Jun-Pyo; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In a closed loop vapor compression cycle, a small portion of the oil circulates with the refrigerant flow through the cycle components while most of the oil stays inside the compressor. The worst scenario of oil circulation in the refrigeration cycle is when large amounts of oil become logged in the system. Each cycle component has different amounts of oil retention. Because oil retention in refrigeration systems can affect performance and compressor reliability, it receives continuous attention from manufactures and operators. Thus, the objective of this dissertation is to develop and use a method to experimentally and theoretically investigate the oil retention behavior in a refrigeration system on a component by component level. The test facility for the oil retention study mainly consists of a refrigeration loop and an oil loop. An oil injection-extraction method was developed to measure the oil retention at each component of the cycle. As the oil circulation ratio increases, the oil retention volume in the heat exchanger and suction line also increases. 16% and 10% of the total oil amount charged initially is retained in heat exchangers at 5 wt.% of oil circulation ratio for the refrigerant mass flux, 290 kg/m2s and 414 kg/m2s, respectively. The effect of oil on pressure drop was found to be most profound at high vapor qualities where the local oil mass fractions are the highest. An analytical model for the annular flow pattern to estimate the oil retention was developed. According to the analysis of CO2 and oil flow in the suction line, the interfacial friction factor should be expressed as the function of CO2 gas Reynolds number as well as the dimensionless oil film thickness. Furthermore, an empirical interfacial friction factor based on experimental results was developed. All simulation results for the suction line are bounded by ± 20% from experimental results. In the case of heat exchangers, void fraction models were used to estimate the oil retention. Due to the changing oil properties, the heat exchangers were divided into segments. Then the oil retention volume in the heat exchangers was calculated from the oil fraction and the length of the corresponding segment. Void fraction models by Hughmark (1962) and Premoli et al. (1971), show good agreement with current experimental results of oil retention at the evaporator and the gas cooler, respectively. Simulation results at the evaporator and the gas cooler are bounded by ± 20% of experimental results. To minimize the oil retention in system components, several design guidelines are suggested.
Development of a Simulation and Optimization Tool for Heat Exchanger Design
(2003) Jiang, Haobo; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.); Mechanical Engineering
Heat exchangers have been used extensively and play an important role in the capital cost, energy efficiency and physical size of refrigeration and air conditioning systems. In this dissertation, a simulation and optimization tool to improve effectiveness and efficiency in design, rating, and analysis of air to refrigerant heat exchangers including conventional finned tube coils and emerging microchannel heat exchangers, Coil Designer, is developed and investigated using a general-purpose modeling concept and user-friendly interface. It is applicable to design of condensers, evaporators, and heating and cooling coils under any operating conditions. A network viewpoint was adopted to establish the general-purpose model and allow for analysis of arbitrary tube circuitry and mal-distribution of fluid flow inside the tubes. Comprehensive evaluation of solutions to the highly nonlinear system of equations in the local thermal/hydraulic performance within the tube network was conducted and a new solution method to successively approximate the physics of heat and fluid flow was developed to enhance the solution convergence capability. A segment-by-segment approach within each tube was implemented, to account for two-dimensional non-uniformity of air distribution across the exchanger, and heterogeneous refrigerant flow patterns through a tube. A further sub-dividable-segment model was created in order to address the significant change of properties and heat transfer coefficients in the single-phase and two-phase regime when a segment experiences flow regime change. The effectiveness-NTU method for cross-flow configuration was used also for combined heat and mass transfer problem under dehumidification, by defining equivalent thermal resistance and heat capacity. Object-oriented programming techniques were applied in developing Coil Designer to facilitate flexible and customizable design platform and building graphic user-friendly interface. Coupled heat exchangers with multiple fluids inside different subsets of tubes can be modeled and analyzed simultaneously. A wide variety of working fluids and correlations of heat transfer and pressure drop are available at the user’s choice. The tabular and graphic representation of performance simulation results provides convenience in comprehensive and detailed parametric analysis. The model prediction with Coil Designer was verified against experimentally determined data collected from a number of sources. The simulation tool was shown to be able to predict the heat transfer rate for a variety of coils with good accuracy. Parametric studies were conducted to confirm the capability of the program in exploring all aspects of heat exchanger performance under a wide variation of design and operating conditions. A genetic algorithm is introduced and integrated with the simulation tool for single and multi-objective optimization design of heat exchanger to accomplish the following goals quickly and accurately: achieve optimum circuitry selection, minimize volume, minimize the amount of material utilized in the coil and thus minimize overall cost of the coil while achieving the best possible performance.
Reliability Assessment of Optical Fibers under Tension and Bending Loads
(2003-10-29) yang, yubing; Barker, Donald; Mechanical Engineering
The mechanical failure of optical fibers must be avoided to ensure reliability of fiber-based systems. The first stress event in a fiber's lifetime is the proof test. The proof test will alter the fiber's strength distribution for all subsequent processing and applications. Thus it is critical to know the fiber strength distribution after proof test (post-proof strength distribution). It is generally assumed that the proof test truncates the strength distribution at the proof test stress level. But, many users are concerned because they know that theoretically it has been shown that after proof test the strength of fiber may be much less than the proof test stress level, and that the minimum post-proof strength is determined only by the unloading rate during the proof test. But this theoretical result is not consistent with historical field data. Historically no one has documented failures stresses below the proof stress level. This dissertation resolves this apparent contradiction by reviewing the theory and conducting a probabilistic assessment. As optical fibers are used more and more in computer and switching gear backplanes, a new potential mechanical reliability problem arises due to the necessary bends introduced in optical fibers. Previous researchers were concerned with the uniform stress optical fibers saw in long haul underground applications, but bending places a non-uniform stress along the fiber surface. So it is inaccurate to borrow fiber usage mechanical guidelines from long-haul application. This dissertation reviews existing theories and then develops a new analytic approach to assess the mechanical reliability of optical fibers under bending loads and static fatigue conditions. This new analytic approach is verified through a simple static two-point bend experiment. Finally the newly developed reliability assessment method is used to develop new guidelines for bending application and examples are presented to show how the approach can be used to attack some very common mechanical reliability problems with optical fibers.

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