No description available.

No description available.

This work presents direct and large eddy simulations of a wide range of heat augmentation surfaces roughened by modified dimples/protrusions. The dissertation is composed of two main parts: Part I (Chapters 2-4) for compact heat exchangers and Part II (Chapter 5) for internal cooling of rotating turbine blades. Part I consists of three phases: Phase I (Chapter 2) investigates flow structure and heat transfer distribution in a channel with dimples/protrusions; Phase II (Chapter 3) studies the application of dimples as surface roughness on plain fins; and Phase III (Chapter 4) considers a new fin shape, the split-dimple fin, that is based on modifying the conventional dimple shape. Chapter 2 presents direct and large eddy simulations conducted of a fin bank over a wide range of Reynolds numbers, ReH=200-15,000, covering the laminar to fully turbulent flow regimes and using two channel height geometries. While the smaller fin pitch channel has better performance in the low to medium Reynolds number range, both channel heights show similar trends in the fully turbulent regime. Moreover, analysis of the results shows that vortices generated in the dimple cavity and at the dimple rim contribute substantially to heat transfer from the dimpled surface, whereas flow impingement and acceleration between protrusions contribute substantially on the protrusion side. Chapter 3 considers applying dimples as surface roughness on plain fin surfaces to further enhance heat transfer from the fin. Three fin geometries that consider dimple imprint diameter effect and perforation effect are considered. The dimple imprint diameter has a minimal effect on the flow and heat transfer of the fin. However, the introduction of perforation in the dimple significantly changes the flow structure and heat transfer on the dimple side of the fin by eliminating recirculation regions in the dimple and generating higher intensity vortical structures. Chapter 4 presents a novel fin shape, the split-dimple fin, which consists of half a dimple and half a protrusion with an opening between them. The split dimple provides an additional mechanism for augmenting heat transfer by perturbing continuous boundary layer formation on the fin surface and generating energetic shear layers. While the protruding geometry of the split dimple augments heat transfer profoundly, it also increase pressure drop. The split dimple fin results in heat conductance that is 60–175% higher than a plain fin, but at a cost of 4–8 times the frictional losses. Chapter 5 studies the employment of dimples/protrusions on opposite sides for internal cooling of rotating turbine blades. Two geometries with two dimple/protrusion depths are investigated over a wide range of rotation numbers, Rob=-0.77 to 1.10. Results show that the dimple side is more sensitive to the destabilizing forces on the trailing surface, while both react similarly to the stabilizing effect on the leading side. It is concluded that placing the protrusion on the trailing side for low rotation number, |Rob|<0.2, provides better performance, while it is more beneficial to place the dimple side on the trailing side for higher rotation numbers. / Ph. D.

Thermal management has become a major concern in the design of current and future more and all electric aircraft (M/AEA). With ever increasing numbers of on-board heat sources, higher heat loads, limited and even decreasing numbers of heat sinks, integration of advanced intelligence, surveillance and reconnaissance (ISR) and directed energy weapons, requirements for survivability, the use of composite materials, etc., existing thermal management systems and their components have been pushed to the limit. To address this issue, more efficient methods of thermal management must be implemented to ensure that these new M/AEA aircraft do not overheat and prematurely abort their missions. Crucial to this effort is the need to consider advanced heat exchanger concepts, comparing their designs and performance with those of the conventional compact exchangers currently used on-board aircraft thermal management systems. As a step in this direction, the work presented in this thesis identifies two promising advanced heat exchanger concepts, namely, microchannel and phase change heat exchangers. Detailed conceptual design and performance models for these as well as for a conventional plate-fin compact heat exchanger are developed and their design and performance optimized relative to the criterion of minimum dry weight. Results for these optimizations are presented, comparisons made, conclusions drawn, and recommendations made for future research. These results and comparisons show potential performance benefits for aircraft thermal management incorporating microchannel and phase change heat exchangers. / Master of Science

The purpose of this investigation was to determine the corrosion-resistant properties of commercial magnesium-alloy tubing as compared to the corrosion resistance offered by silicone-coated magnesium-alloy, aluminum, monel and stainless steel when used as heat exchanger tubes. Two single pass, double pipe heat exchangers were constructed using pyrex glass tubes as outer shells. The pyrex tubes were two inches inside diameter, 2.25 inches outside diameter, five feet long and had flared ends. Each end of the pyrex tubes was equipped with a flange containing a 0.876 inch packing gland and a 0.75 inch inlet, or outlet port. The corrosion test tubing could be easily inserted into or removed from the packing glands without damaging the surface of the test specimen. A series of corrosion tests was made in the heat exchangers using magnesium-alloy, FS-1; silicone-coated magnesium-alloy, FS-1; aluminum, 3S; stainless steel, type 316; and monel as heat exchanger tubes. Three and ten per cent by weight sodium chloride solutions, and sulfur-bearing Texas fuel oil were used as corrosive mediums. The unit was operated at an average inlet temperature of 38 ± 1°C and an average outlet temperature of 50 ± 5°C for the sodium chloride solutions, and an average inlet temperature of 83 ± 3°C and an average outlet temperature of 94 ± 6°C for the sulfur-bearing fuel oil. Seventy-two-hour tests were made maintaining an average rate of flow of corrosive medium through the heat exchangers of 6.7 ± 0.2 gallons per minute for the sulfur-bearing fuel oil. Upon completion of the tests, the heat exchanger tubes were chemically cleaned of corrosion products. From the known weight losses, area and density of test specimens, and duration of tests, the corrosion rates were calculated. Corrosion rates expressed in inches penetration per year, due to the action of three per cent by weight sodium chloride was as listed in the following descending order: magnesium-alloy, FS-1, pitted, no calculations made; silicone-coated magnesium-alloy, FS-1, 0.1388; aluminum, 3S, 0.0508; monel, 0.0050; and stainless steel, type 316, 0.0013. Corrosion, expressed in inches penetration per year, due to action of ten per cent by weight sodium chloride was as follows: magnesium-alloy, FS-1, and silicone-coated magnesium-alloy, FS-1, pitted, no calculations made; aluminum, 3S, 0.0588; monel, 0.0071; stainless steel, type 316, 0.0036. Corrosion, expressed in inches penetration per year, due to action of sulfur-bearing fuel oil was as follows: silicone-coated magnesium-alloy, FS-1, 1.160; magnesium-alloy, FS-1, 0.897; aluminum, 3S, 0.0361; monel, 0.0095; stainless steel, type 316, 0.0029. <i>Note: After completion of this thesis, an investigation by John T. Castles concerning the use of silicone coating on steel evaporator tubes indicated that the coating contained minute holes and therefore was not impervious. These holes could have formed at points of weakness and starting points for disintegration of the silicone coating. It is recommended that the metal surface be treated prior to spraying in an attempt to obtain an impervious silicone coating. The metal surface should be thoroughly dried to insure against moisture remaining in nonconformities which would establish points of weakness under the coating. Some test should be devised which would indicate whether or not a coating was impervious.</i> / Master of Science

Hypersorption was recently developed by the Union Oil Company of California. The hypersorption process consists of a moving bed of an adsorbent passing counter currently to the gaseous flow. The gases are separated by selective adsorption. Of basic importance for successful operation is the heating of the adsorbent to obtain desorption. Considerable difficulty in achieving this transfer of heat is encountered because of the non-conducting nature of the adsorbent. The purpose of this investigation was to determine the heat transfer characteristics for the design of an improved heat exchanger for a moving bed system composed of air and activated carbon. This investigation was made studying the transfer of heat to five sizes of activated carbon, from 0.078 inches to fines, at flow rates of 2.2 to 11.0 pounds per hour. Steam at pressures of 15 to 75 pounds per square inch, gage, was used to heat the carbon moving through a three-quarter inch black iron pipe 36 inches long with an effective heating area of 0.65 square foot. The results of this investigation showed that the desorption of moisture in the activated carbon caused an unexpected break in the thermal conductivity curves between 200 and 220 °F, resulting in variable thermal conductivity-specific heat ratios. Rod-like flow was evidenced through e physical test, but poor correlation was observed between the date and the rod-like equation• The over·all heat transfer coefficient varied from one to three Btu per hour—square foot-°F per foot, increasing as the carbon flow rate and the Graetz number, KL/Wcp, increased. / Master of Science

This thesis contributes an in-depth comparative study of u-tube and coaxial borehole heat exchangers. While it is widely accepted that the lower resistance of the coaxial heat exchanger should result in a performance advantage, the findings of several studies comparing the heat exchanger configurations did not definitively establish the mechanisms causing differences in performance. This study employs numerical modelling to consider heat exchangers over a broad range of time scales and under carefully controlled geometry and flow conditions, resulting in the identification of the key parameters influencing borehole heat exchanger performance. The first part of this study consists of a comparison of u-tube and coaxial heat exchangers under continuous loading. A detailed conjugate heat transfer numerical model was developed in OpenFOAM, designed to capture both short and long time scales of heat exchange, necessary to understand the nuanced differences between designs. A novel transient resistance analysis was employed to understand the dominant factors influencing performance. This study established that marginal differences exist between u-tube and coaxial borehole heat exchangers (BHEs) when operated continuously long term but that greater differences occur early in operation. The second phase of this investigation provided a framework for analysing borehole heat exchanger performance during intermittent operation, while also comparing u-tube and coaxial designs. During this study, it was found that reducing operating time, improving the the rate of the ground's recovery to its original temperature, and lowering the duty cycle improved BHE performance. Transit time was identified as a influential time scale, below which heating at the outlet was limited. Further, the benefits of operating below the transit time were mitigated by design-specific interaction between inlet and outlet flows. Finally, this study found that non-dimensionalizing operating time by transit time causes the differences between u-tube and coaxial performance to vanish, leading to the conclusion that differences in BHE performance are caused by variations in flow rather than thermal mass. / Thesis / Doctor of Philosophy (PhD) / This thesis provides an in-depth comparative study of two different designs of borehole heat exchanger, the u-tube and coaxial, which are used in geothermal applications to transfer heat to and from the ground. While many researchers anticipated that the coaxial design would perform better, several studies comparing the heat exchangers were not able to provide a clear answer about which heat exchanger performed best. This study addressed this gap by using detailed numerical simulations which showed that there was a marginal difference in performance between the two heat exchangers when operated for periods longer than a few hours, but that larger differences occurred early in operation (under 15 minutes). The results also showed that operating intermittently resulted in improvements in performance of the heat exchanger, particularly when operated for periods less than the time it takes fluid to travel the length of the piping.

The object of this thesis was to determine water film coefficients and condensing steam film coefficients for a single tube heat exchanger. A shell and tube apparatus was constructed and these coefficients were determined by Wilsons graphical method. Test runs were made at various pressures and water velocities. It was determined that for flow through a horizontal tube the water film coefficient closely approximates 416 V_w^0.8 Btu/hr-ft²-F, where V_w equals water velocity in feet per second, and the Reynolds number lies between 17,000 and 100,000. It was also found that an average condensing steam film coefficient for filmwise condensation was 2000 Btu/hr-ft²-F. It was discovered that this coefficient is much higher if the condensing surface is highly polished. / Master of Science

Metal foams have many attractive properties such as light weight, low relative density, energy absorption capability etc. One of the main advantages of metal foam is that the foam inherits several properties of the parent metal, at the same time, at a fraction of the weight. Metal foams are basically of two types; closed pore and open pore. In the open pore configuration the highly porous structure with large surface to volume ratio is attractive in thermal applications such as heat exchangers, small scale refrigeration, diesel exhaust cooling and heat sink for electronics. Large surface area to volume ratio of the heat transfer area is an important parameter in design of heat exchangers. Application of open cell metal foam as a heat exchanger involves production of the metal foam, cutting/drilling the metal foam to required dimensions and attaching it to a substrate or duct. Foams are cut by various methods such as by using circular saw, band saw, abrasive sawing wire or electrical discharge machining. Cutting or drilling operations plastically deform the struts and affect the surface roughness of the struts and hence, the contact area between the foam and the substrate. The foam and the substrate are then joined to get the final product. Various techniques are adopted to join the foam and substrate that includes, press fit, welding, soldering, brazing and use of epoxy adhesives or thermal glue. These methods either deform the foam plastically or involve a bonding material which involves an additional step in manufacturing and is generally necessary to reduce the thermal resistance at the interface. Every secondary step involved in machining the foam and joining it to substrate/duct add to the energy, time and cost of the component. Significant amount of materials wastage occurs during the production and machining steps of the metal foam. Bonding material used for attaching foam to the substrate makes the recycling of the heat exchangers difficult. In the present research work the above issues were rectified by introducing a novel method of fabricating the heat exchanger in a single step. This can be done by producing open cell foam, bonded to the substrate in a single step to get the ready to use heat exchanger. The uniqueness of the method/ process is that it provides an advantage of manufacturing heat exchangers consisting of open cell aluminium foam both inside and outside the aluminium duct/substrate. Here open cell metal foam is metallurgic ally bonded to the aluminium duct without producing any distortion in the aluminium duct. The present method avoids the secondary cutting and joining operations, hence reducing material and energy wastage. This heat exchanger does not need a bonding material at the foam duct interface which makes the product completely recyclable without even having to separate the aluminium foam and, many-at-times, the copper substrate. Further, in the present process no hazardous material is involved in the fabrication process of the heat exchanger and all the materials used for the foam production can be recycled. Another unique advantage of this process is that the foam can also be cast inside and outside the tube in a single step. This helps increase the heat transfer area per unit volume inside the tube increasing the effectiveness significantly. First, an attempt was made to cast aluminium foam over a Cu substrate. Spheres made of Plaster of Paris (PoP) were used as space holders to create pores in the foam. First, a dough of PoP was prepared by mixing sufficient amount of water with the powder of PoP. Small pieces of PoP were taken from the dough and were rolled by hands to prepare spherical balls. Next, a casting setup was made where a die made of stainless steel was placed in a crucible whose bottom was filled with sand. A tube/duct made of copper was placed at the centre of the die and PoP balls were dropped around the duct. This setup was then placed in a furnace and was preheated to remove all the moisture from the PoP. Molten aluminium at around 700 °C was poured into the preheated die. After solidification, the die was opened and cast was allowed to cool in ambient air. PoP balls were removed by using a sharp needle and by dipping the casting in acetic acid. After removal of PoP from the cast, interconnected holes/cavities formed in the place of space holders/PoP balls, forming pores in the foam. There are some limitations of this method such as removal of PoP was tedious and needed chemicals that need to be discarded, PoP cannot be recycled and creates waste, small amount of moisture present in PoP balls can cause an explosion. The bonding between aluminium foam and Cu substrate obtained was not good, giving rise to thermal contact resistance. Due to the above limitations further implementation of this process using PoP was not explored further. There was a need of space holder material which can withstand the temperature of molten Al and also can be removed easily from the cast without any use of chemicals. Obtaining metallic bonding between foam and Cu substrate was difficult due to the corrosion layer formation at the interface of Al and Cu substrate due to preheating. If preheating was not carried out full penetration of the molten aluminium did not take place in the space available in between the spheres. Therefore, it was decided to cast Al foam over Al substrate. The main challenge and difficulty was to cast open cell Al foam inside and outside the tube/duct made of the same material (Al) without distorting the tube/duct as well as achieving consistent metallic bonding between the two. This has been successfully done by gravity casting method a single step manufactured and ready to use open cell Al foam heat exchanger were fabricated. A casting setup was prepared, which consisted of a commercially pure aluminium tube placed in the middle of a stainless steel split die. The gap between the tube and die was filled with the salt spheres. An uncommon and new approach was adopted to produce NaCl salt spheres. NaCl salt balls (spherical and ovoid) of different diameters were processed by casting route. The casting step of NaCl is necessary as the moisture present in NaCl can be completely removed during the melting of NaCl. NaCl was chosen as it had a melting point higher than aluminium. The casting setup was placed in a furnace and was preheated to various temperatures up to 550 °C. Commercially pure aluminium was melted separately in a crucible and was poured into the steel die at 700oC. The liquid metal flows through the die and fills the cavities between the salt balls. The die was opened immediately after solidification of molten Al and cast was allowed to cool in ambient air. The salt (NaCl), which was still solid, was dissolved in water to get the foam structure. With proper control of the preheat temperature and temperature of liquid aluminium no distortion of the aluminium duct was observed throughout the length of the heat exchanger. Consistent and complete fusion/ metallic bonding was observed at the interface of Al foam and Al substrate/duct. Several heat exchangers with different porosity and pore geometry with the aluminium foam cast outside the tube and both inside and outside of the tube were fabricated. The beauty of the designed method is that it is simple and cost effective and eliminates the major issue of thermal contact resistance since the foam and the duct are made of the same material and are bonded in the liquid state leaving no interface between the foam and the duct. Further, foam can also be cast inside the duct in the same step while casting the foam outside the tube, giving an integral heat exchanger which has higher heat transfer surface area to volume ratio inside and outside the duct. This is expected to further improve the efficiency and effectiveness of the heat exchanger An added advantage of this method is that the heat exchanger can be recycled easily in a single step re-melting route. Further, the heat exchanger does not use any hazardous material during manufacture that needs attention during recycling. After the production and fabrication of the heat exchangers, the thermal performance or effectiveness of the heat exchangers was assessed, to evaluate its usefulness and suitability for heat transfer application. An experimental test setup was fabricated in the laboratory to perform the heat transfer tests. The experimental test setup consists of the following major components;1) A test chamber whose function was to insulate the heat exchangers from the surroundings and to avoid any heat loss to the surroundings, 2) An air blower used to supply cold fluid (air) to the test chamber, 3) A constant temperature bath was used to supply the hot fluid, which was water in this case, in the duct of the heat exchanger, 4) A rotameter was used to measure the volumetric flow rate of the cold fluid and 5) A pressure gauge having the pressure measurement range between 1 mbar to 160 mbar to measure the pressure drop across the test chamber. K-type chromel – alumel thermocouples having temperature measurement range between -270 °C to 1,260 °C were used to measure the temperature of hot and cold fluids during the experiments. By aid of the data logger system and computer, temperature readings were recorded during the tests and were used further for the heat transfer calculations. For testing the aluminium foam heat exchangers was placed in the insulated test chamber. Hot water was supplied inside the duct of heat exchanger whereas air at room temperature was supplied around the foams at varying flow rates during the tests. During the tests, temperature readings were taken at steady state condition. NTU-Effectiveness method was used to evaluate the thermal performance of heat exchangers. Overall results obtained by this experimental study are as follows • As the inlet temperature difference between hot and the cold fluids increases the heat transfer rate and the effectiveness of the heat exchangers also increases. • At a constant flow rate of hot fluid, heat exchangers exhibits significantly better thermal performance at lower flow rate of cold fluid compared to higher flow rate. As the flow rate of cold fluid increases, the velocity of the fluid increases and consequently, reduces the optimum interaction time between hot and the cold fluids required for the efficient heat transfer. • At a constant and low flow rate of cold fluid the effectiveness of the heat exchanger increases as the porosity of the foam increases. But when the flow rate of cold fluid was increased further after a certain limit, the effectiveness value of the heat exchanger decreases. • Heat exchanger consisting of foam of higher porosity exhibits higher effective. • Heat exchanger having foam inside and outside of the duct/tube exhibits significantly higher effectiveness compared to Al duct, Cu duct and other heat exchanger tested. • At a higher flow rate of the cold fluid, the heat exchangers consisting of foams of higher porosity, experience more drop in effectiveness compared to the heat exchanger having foams of low porosity. • Pressure drop across the length of the foam/fin increases as the volumetric flow rate of the cold fluid (m3/s) increases. • Surface area per unit volume and effectiveness values for bare Al tube is very low compared to Al foam heat exchangers resulting in the bare Al tube exhibiting much lower effectiveness compared to heat exchanger made of Al foam. • For a certain flow rate of fluids, the effectiveness of the heat exchanger increases up to a certain thickness of the Al foam. • Regardless of the thickness of the foam, the effectiveness of the heat exchangers is low at higher flow rate of cold fluid compared to lower flow rate. • These foam based heat exchanger had a much higher effectiveness when compared to that of other heat exchangers, data of which were got from literature. The present experimental study concludes that fuse bonding open cell aluminium foam over an Al duct or Al substrate can improve the thermal performance of the heat exchanger significantly. The thesis includes five chapters. Chapter 1 gives a detailed introduction about the metal foam, heat exchangers, thermal contact resistance and its effect on the heat transfer rate has been explained. This chapter also includes the overall aim and motivation for the research work. Chapter 2 covers the literature available on production methods of metal foam and its limitations has been listed out. And conventional methods of manufacturing open cell metal foam heat exchangers and its disadvantages have been explained in detailed. Chapter 3 covers in detail the novel method of production and fabrication of open cell metal foam heat exchangers. Chapter 4 includes an experimental study, where thermal performance of heat exchangers has been assessed through heat transfer experiments. Chapter 5 is the conclusions and future works.

Call number: LD2668 .T4 1979 H43 / Master of Science