Approved for public release; distribution is unlimited. / This research presents the results of a combined numerical and experimental study of heat transfer and pressure drop behavior in a compact heat exchanger (CHE) designed with drop-shaped pin fins. A numerical study using ANSYS was first conducted to select the optimum pin shape and configuration for the CHE. This was followed by an experimental study to validate the numerical model. The results indicate that the drop shaped pin fins yield a considerable improvement in heat transfer compared to circular pin fins for the same pressure drop characteristics. This improvement is mainly due to the increased wetted surface area of the drop pins, and the delay in the flow separation as it passes the more streamlined drop shaped pin fins. The data and conclusions of this study can be used in heat exchanger design for large heat flux cooling applications as in gas turbine blades, and high-power electronics. / Lieutenant Junior Grade, Tunisian Navy

No description available.

In this thesis a novel computer based model is developed which accurately simulates the operation of a plate heat exchanger (PHE). The model allows for the variation of all relevant fluid properties as the temperatures of the fluids vary through the PHE. It is set up in a spreadsheet in such a way that one can observe the variation of fluid properties and heat transfer parameters through the PHE during steady state operation. Although the model could be used for general purpose analysis of PHE's, it is intended to be used in an educational environment, where students can run "virtual lab sessions" with the model and so gain a better understanding of the overall and detailed operation of plate heat exchangers. The model is validated using experimental data representing a range of different PHE sizes, flow configurations, fluid types and flow conditions. Instructions have been provided on how it can be used in an educational environment to assist student to discover more about the general and detailed operation of a PHE.

The performance of membrane heat exchangers for HVAC total energy recovery systems was evaluated through experimentation and detailed system modelling. The operating principle of the membrane heat exchanger (enthalpy heat exchanger) is based on passing ambient hot and humid supply air over one side of a porous membrane heat exchanger surface and cold and less humid room exhaust air on the other side of the transfer surface. Due to the gradient in temperature and vapour pressure, both heat and moisture are transferred across the membrane surface causing a decrease in temperature and humidity of the supply air before it enters the evaporator unit of the conventional air conditioner. Hence both sensible and latent energy are recovered. In this study, both experimental and numerical investigations were undertaken and mathematical models were developed to predict the performance of the latent heat recovery heat exchangers for use with conventional air conditioning systems. The membrane moisture transfer resistance was determined by a laboratory-scale permeability measurements. It was found that the membrane heat exchanger performance is significantly influenced by the heat exchanger flow profile and shape, heat and moisture transfer material characteristics, air velocity and air moisture content. Improvement of membrane heat exchanger performance requires an in depth study on flow, temperature and moisture distribution in the heat exchanger flow paths. Thus, a commercial CFD package FLUENT is used to model the membrane heat exchanger. However, software of this type cannot model moisture diffusion through the porous transfer boundary. Therefore, two user defined function codes have been introduced to model the moisture transfer in latent energy heat exchangers. The annual energy consumption of an air conditioner coupled with a membrane heat exchanger is also studied and compared with a conventional air conditioning cycle using the HPRate software. Energy analysis shows that in hot and humid climates where the latent load is high, an air conditioning system coupled with a membrane heat exchanger consumes less energy than a conventional air conditioning system. The membrane heat exchanger dehumidifies the air before it enters the air conditioning system, resulting in a decrease in energy consumption in comparison with conventional air conditioning system.

Thermal contact resistance or, its reciprocal, thermal contact conductance is an important parameter in a wide range of thermal phenomena. It plays a significant role in heat transfer applications such as electronic packaging and nuclear reactors. This parameter also appears in fin-tube heat exchangers; however, it is often neglected in the performance calculations of heat exchangers. This thesis project explores the means by which the heat transfer performance of a finned tube heat exchanger may be enhanced. It includes experimental studies and finite element analysis investigating the effects of expansion bullets and coatings on the thermal contact conductance. An apparatus has been designed and fabricated for the experimental part of the work. A finite element model established the fintube configuration to be used in the design and manufacture of the apparatus. The apparatus was specially made for measuring thermal contact conductance directly in a finned-tube heat exchanger both in vacuum and in gaseous environment. The experiments were done on hexagon shaped specimens with a single fin connecting seven tubes. Sixteen type-T thermocouples have been used to measure temperatures at several locations on the specimen. A full-scale quarter-fin model was chosen for a second finite element analysis. The model simulates the actual specimen and predicts the temperatures. The finite element analyses have been used to validate the experimental results. The experimental results from the bare contact specimens, assembled with different sizes of expansion bullet, show that while higher expansions enhance the thermal contact conductance, the effect of interstitial gas such as nitrogen is beneficial for all specimens expanded with the 9.42 mm size bullet. Applying a coating material with high thermal conductivity is also an effective way to enhance the thermal contact conductance. The results show that the highly conductive plating materials, such as zinc, tin, silver and gold, enhance the thermal contact conductance. The presence of interstitial gas such as nitrogen also results in higher heat transfer rates and higher thermal contact conductance compared to those obtained in vacuum.

Energy is an essential and critical commodity and our reliance on it has fuelled much of the debate and interest in society and academia alike. Environmental concerns, depleted energy resources and higher energy prices are the main factors that drive this interest. Energy efficiency is one of the main avenues to preserve and better utilize this valuable commodity. The energy exchange by employment of heat exchangers is extensive and tube-fin heat exchangers are widely used in industrial and commercial applications. Smarter designs could not only improve energy efficiency but may also save on material costs. Although mass production and improved manufacturing techniques have reduced manufacturing costs, tube fin heat exchangers have not evolved greatly to take advantage of these improvements. There has been a large range of fin surface enhancements proposed, such as waffled fins or louvres and while limited improvements in capacity have been achieved, this is generally accomplished at a much larger pressure drop penalty. Numerous studies have been performed in order to examine the potential of various surface enhancement geometries on an ad hoc basis. These presumably operate on the basis of enhanced convection due to increased turbulence levels. However very few of these studies examine the actual nature of turbulence that is responsible for convection enhancement. A series of experiments and numerical studies have been conducted to quantify the effect of the turbulence vortex characteristics on the air side heat convection of a tube-fin heat exchanger. Homogeneous, transverse and streamwise vortical structures were investigated. The thermal transfer performance resulting from these flows was compared to that of standard louver fin geometries by considering sensible heat transfer only, applicable to radiator applications. Several novel coils designed to achieve these vortex structures, were developed and their heat transfer characteristics were quantified. These coil designs can be described as the Tube Mesh, Tube Strut and a Delta-Winglet fin surface.The Tube Mesh heat exchanger consisted entirely of horizontal and vertical tubes arranged in an approximate homogeneous turbulence generating grid. While they had a lower heat transfer of between 53% to 63% of that of the louvre fin surface, they had an extremely low pressure drop of 25% to 33%. This has the potential to make them suitable for certain low pressure drop applications, especially if energy saving is a prerequisite. The range of Tube Strut coils consisted of a tube bundle with interconnecting heat conducting struts to form a parallel plate array were also investigated. Three different strut thicknesses and strut spacing were trialled. In general these had similar performance to the tube mesh at 45% to 65% the heat transfer capacity of the louver fin surface. The resulting pressure drop was 38% to 42% of that of the louver fin surface. A delta-winglet design which positioned the deltas in a flow up configuration just in front of the tubes was examined. It was found that this configuration had an almost comparable capacity of 87% to a louver surface having the same fin pitch. On the other hand it had approximately half the pressure drop of 54% of the similar louver fin surface. This particularly low pressure drop makes this design preferable from an energy utilisation perspective. While a slight increase in coil area is required, this is offset by an almost 50% reduction in operating costs by reducing the parasitic energy requirements of the convection fans. The experimental data gathered for this Delta-Winglet design served to validate a succession of numerical simulations which were performed to estimate the performance of other configurations of multiple vortex generators. In addition the performance of combining a delta-wing with a louvred surface was investigated. It was found that increasing the number of delta-winglets or combining deltas with a louvred surface provided little improvement in heat transfer but increased pressure drop substantially. The louvre design itself was examined, and simulations were undertaken to estimate the effect of louvre angle, as well as louvre pitch. A hitherto unexamined concept was to investigate the effect of having louvres with serrated edges. It was found that an increase in louver angle by 5 degrees had negligible effect on heat transfer but increased the pressure drop by 17%. A variation in louver pitch showed a minimal variation in both heat transfer and pressure drop. Surprisingly a serrated louver showed a slight reduction in both heat transfer and pressure drop but this was miniscule. It was established throughout the course of the investigations that the bulk of the coil heat transfer is performed by the first tube row. Therefore the potential for increasing heat transfer by shifting some heat exchange to the down stream rows was examined. This was attempted by having progressively increasing louvre angles from the front of the coil to the rear. While a slight increase in heat transfer performance was achieved, this accomplished at the expense of a 13%-14% increase in pressure drop. The outcomes have shown that substantial net improvement of heat exchanger energy efficiency can be achieved through optimization of the turbulence generation along the fins of a tube fin heat exchanger. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1253254 / Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2006

Material failures in heat exchangers are often closely tied to events associated with the conditions of service and operating parameters. These events can generally be attributed to adverse load application and higher than optimum operating temperatures that could lead to changes in the microstructure of the materials and fatigue failure of the component. However, fatigue failure in heat exchangers is usually associated with the presence of a biaxial stress condition. Two nonparallel forces create a two-dimensional stress field at the free surface of the structural element where the process and mechanism of fatigue failure normally initiate. An experimental investigation was conduct6d to evaluate the biaxial fatigue behavior of commercially pure titanium Ti-50A (Grade 2) and low-alloy titanium Ti-Code 12 (Grade 12) heat exchanger materials. The biaxial state of stress was composed of an axial stress and a superimposed torsional stress, applied in a thin-wall tubular specimen machined from titanium tubing. Torsional stress was applied independently using a torsion machine and a torque fixer assembly devised as part of this study. After applying the desired torsion, the torsionally stressed specimen was mounted on a closed-loop electrohydraulic machine for the application of axial cyclic loading. A minimum of four tests were conducted for each of three alternating stress levels at both high and low torsional stresses. The biaxial fatigue test under load control condition was done under fully reversed cycles equivalent to a biaxiality ratio of -1. These test parameters were determined from an analytical formulation based on Mohr's circle. The results are presented in terms of the various measured or calculated quantities versus number of cycles to fracture. Biaxial fatigue curves were drawn through the experimental points corresponding to Weibull's mean life criterion. The four data points exhibit scatter that appears to be related to the applied stress amplitude. It was also found that a correlation exists between the magnitude of applied cyclic biaxial stress and fatigue life to failure. In addition, the results have been discussed taking existing failure criteria into account. / Graduation date: 1985

Thesis (M.S.)--Ohio State University, 1983. / Includes bibliographical references (leaves 84-85). Available online via OhioLINK's ETD Center

Thesis (M.S.)--Virginia Polytechnic Institute, 1952. / Typewritten. Bibliography: leaf 56. Also bibliographical footnotes. Also available via the Internet.

Thesis (Mechanical Engineer and M.S. in Mechanical Engineering)--Naval Postgraduate School, June 2003. / Thesis advisor(s): Ashok Gopinath. Includes bibliographical references (p. 73-74). Also available online.