Heat transfer is a critical part of engineering design, from the cooling of rocket engines to the thermal management of the increasingly dense packaging of electronic circuits. Even for the most fundamental modes of heat transfer, a topic of research is devoted to finding novel ways to improve it. In recent decades, investigators experimented with the idea of exposing systems to acoustic waves with the hope of enhancing thermal transfer at the surface of a body. Ultrasound has been applied with some success to systems undergoing nucleate boiling and in single-phase forced and free convection heat transfer in water. However, little research has been done into the use of sound waves to improve heat transfer in air. In this thesis the impact of acoustic waves on natural convection heat transfer from a horizontal cylinder in air is explored. An experimental apparatus was constructed to measure natural convection from a heated horizontal cylinder. Verification tests were conducted to confirm that the heat transfer could be described using traditional free convection heat transfer theory. The design and verification testing of the apparatus is presented in this work. Using the apparatus, experiments were conducted to identify if the addition of acoustic waves affected the heat transfer. For the first set of experiments, a 40 kHz standing wave was created along the length of the heated horizontal cylinder. While our expectation was that our results would mirror those found in the literature related to cooling enhancement using ultrasound in water (cited in the body of this thesis), they did not. When a 40 kHz signal was used to actuate the air surrounding the heated cylinder assembly, no measurable enhancement of heat transfer was detected. Experiments were also performed in the audible range using a loudspeaker at 200 Hz, 300 Hz, 400 Hz, 500 Hz, and 2,000 Hz. Interestingly, we found that a 200 Hz acoustic wave causes a significant, measurable impact on natural convection heat transfer in air from a horizontal cylinder. The steady-state surface temperature of the cylinder dropped by approximately 12℃ when a 200 Hz wave was applied to the system.

A study was undertaken to create a finite element model of a cooled throat converging/diverging rocket nozzle to be used as a tool in designing a cooled aerospike nozzle. Using ABAQUS, a simplified 2D axisymmetric model was created featuring only the copper throat and stainless steel support ring, which were brazed together for the experimental test firings. This analysis was a sequentially coupled thermal/mechanical model. The steady state thermal data matched closely to experimental data. The subsequent mechanical model predicted a life of over 300 cycles using the Manson-Halford fatigue life criteria. A mesh convergence study was performed to establish solution mesh independence. This model was expanded by adding the remainder of the parts of the nozzle aft of the rocket motor so as to attempt to match the transient nature of the experimental data. This model included variable hot gas side coefficients in the nozzle calculated using the Bartz coefficients and mapped onto the surface of the model using a FORTRAN subroutine. Additionally, contact resistances were accounted for between the additional parts. The results from the preliminary run suggested the need for a parameter re-evaluation for cold side gas conditions. Parametric studies were performed on contact resistance and cold side film coefficient. This data led to the final thermal contact conductance of k=0.005 BTU/s•in.•°R for contact between metals, k=0.001 BTU/s•in.•°R for contact between graphite and metal, and h=0.03235 BTU/s2•in.•°R for the cold side film coefficient. The transient curves matched closely and the results were judged acceptable. Finally, a 3D sector model was created using identical parameters as the 2D model except that a variable cold side film condition was added. Instead of modeling a symmetric one or two inlet/one or two outlet cooling channel, this modeled a one inlet/one outlet nozzle in which the coolant traveled almost the full 360° around the cooling annulus. To simplify the initial simulation, the model was cut at the barrier between inlet and outlet to form one large sector, rather than account for thermal gradients across this barrier. This simplified nozzle produced expected data, and a 3D full nozzle model was created. The cold side film coefficients were calculated from previous experimental data using a simplified 2D finite difference approach. The full nozzle model was created in the same manner as the 2D full nozzle model. A mesh convergence study was performed to establish solution mesh independence. The 3D model results matched well to experimental data, and the model was considered a useful tool for the design of an oxidizer cooled aerospike nozzle.

The purpose of this thesis is to effectively model the time history of the temperature distribution during the selective laser sintering process and use this information to investigate the resulting relative density. The temperature is a critical parameter of the process because it directly effects the overall quality of the part. First, an efficient, affordable, and reliable simulation was developed within the finite element software, Abaqus. Next, the results from the simulations were compared to the experimental results performed by Wang et al. (2016). The FEA model consisted of a 3 layer simulation. Multiple simulations at various laser recipes were conducted using W-Ni-Fe as the powder material. The P/v (laser power/scanning speed) was plotted against the resulting total time above the melting temperature for various simulation. It was concluded that a linear relationship exists between the P/v parameters used in the laser recipe and the resulting time above the melting temperature. The average R2 values for the W-Ni-Fe simulations for layer 1, 2, 3 were 0.962, 0.950, and 0.939, respectively. Additionally, the experimental results from the Wang et al. (2016) study confirmed that a linear relationship is present. Thus, it can be concluded that the P/v parameters used within the laser recipe has a direct relation to the resulting relative density of the SLS part.

This report details an approach to using metal foam heat exchangers inside an integrated thermal management system on a variable cycle engine. The propulsion system of interest is a variable cycle engine with an auxiliary, variable flow rate fan. The feasibility of utilizing an open-celled metallic foam heat exchanger in the ducting between the constant and variable-fans on this variable cycle engine to cool the avionics was explored using an experimental approach. Two heat exchangers, 6.3 inch width by 6.3 inch length by 0.5 inch thickness, were constructed from 20 and 40 pores per inch (PPI) metal foam and tested. Both were constructed using 6061-T6 aluminum open-cell metal foam with a relative density of 8% and brazed using 4047 aluminum braze to 0.02 inch thick sheet metal made of 6061-T6 aluminum. Both models were subjected to internal forced convection using heated air with flow rates of 4, 8, 12, 16, and 20 standard cubic feet per minute (SCFM). They were also subjected to external forced convection using blowers to supply cooling air to simulate the variable cycle engine’s fans. One duct was supplied with a constant 34 ft/s cooling flow, while the other cooling flow velocity was varied between 0% and 100% of this 34 ft/s, in 25% increments. The temperature and pressure of the flow internal to the metal foam, as well as the heat exchanger external surface and cold flow temperatures, were recorded. A hot-flow Reynolds number range of 1,300 to 6,400 was tested. Results showed expected trends for the hydraulic performance of both heat exchangers. The form factors were 50.4 and 54.8 ft^-1 and the permeabilities were 9.11E-7 and 6.32E-7 ft^2 for the 20 and 40 PPI heat exchangers, respectively. Due to a defect on one side of the 40 PPI heat exchanger, the thermal results are based only on the 20 PPI heat exchanger. While the present study examines a different metal foam heat transfer configuration than most other studies, the metal foam Nusselt numbers were comparable to past studies. In addition, the pumping power required was not excessive and would allow the thermal management system to be realized without an unreasonable energy input. Therefore, a metal foam heat exchanger integrated within the ducting of a variable cycle engine is deemed feasible. The pumping power and thermal resistance were used to create a performance predicting model of the 20 PPI heat exchanger. From this model, the optimized 20 PPI heat exchanger has a hot-flow rate of 10.5 SCFM. The resulting pumping power and thermal resistance are estimated to be 6.7 BTU/hr and 0.036 °R/(BTU/hr), respectively.

The propagation of a shock and flame from a detonation wave injected orthogonally into a combustible, supersonic flow was observed. The detonation wave was generated through the use of a miniaturized detonation tube, henceforth referred to as a predetonator. Conditions within the test section, including stagnation pressure and equivalence ratio, were varied between cases. Through the use of high-speed schlieren, shadowgraph, and broadband OH chemiluminescence imaging, the leading shock and reaction were recorded as they moved through the test section. Variation of stagnation pressure affected the propagation of the leading shock. Higher stagnation pressures caused greater deflection of the shock wave and jet issued by the predetonator. It was seen that at sufficiently high equivalence ratios, the shock and reaction were able to travel upstream from the test section into the diverging section of the converging-diverging nozzle. Shortly after the shock entered the nozzle, evidence of the initiation of shock induced combustion was observed. Stagnation pressure variation in the range tested had little effect on the ability to initiate a reaction. Multiple behaviors of the shock-induced-combustion were observed, dependent upon the equivalence ratio of the flow through the test section. Behaviors include sustained reaction on the edges of the flow, sustained reaction in the core of the flow, and periodic, non-sustained reaction.

This paper examine a passive cooling system for a humid climate. This system will be divided into two parts, a radiative system and an evaporative system combined into a roof pond system. Performance of the radiative system will be enhanced through the use of a selective cover which will make use of an atmospheric window between 8 and 13um. An attempt will also be made to thermally isolate the radiative system from convective gains with the evaporative system. The evaporative system will consist of a water, solvent and dye layer over the selective cover of the radiative system. The performance of the evaporative system will be enhanced by virtue of the increased vapor pressure made available through the use of solvents. The main solvent to be examined shall be methanol. The increased vapor pressure shall sufficiently increase the rate of evaporative cooling to a point where useful cooling is obtained even under high humidity conditions. It was found that a solution with a 0.8 mole fraction of methanol in the evaporative system could cool a sufficiently large water storage to 45°F using a 300 m2 roof pond. This is a heat sink which if used to provide cooling and dehumidification, will provide 576000BTU of cooling. This is the equivalent of a 3 ton unit operating 16 hours a day. It was found that a water layer thicker than 0.1 mm would radiatively isolate the selective cover, making the concept of a liquid thermal protection useless as a means of providing only convective protection. However, as a selective cover, teflon was found to make the best use of the 1-13um window. As a result, this would provide 33 BTU/ft2-might as compared to 11 BTU/ft2-night for a black cover. It was also found that a green of blue and yellow or red dye mixture, when dissolved in water, would provide a black surface throughout the visual and infrared range.

The characterization of flammability parameters such as time to ignition, mass loss rate (MLR), and extinction criteria is critical for understanding ignition and burning behavior of timber products. These parameters, often determined with bench scale experiments, have previously been presented in literature. However, standard test methods generally use relatively low trial quantities (e.g., n=3) which can potentially cause large variation in reported values. This study investigates the influence of trial quantity on observed statistical variation in key flammability metrics for timber products (e.g., ignition time, peak MLR, MLR at extinction). Using a conical heater, 100 repeat trials were conducted at incident heat exposures of 20 kW/m2, 40 kW/m2, and 50 kW/m2 on 12.7 mm thick ACX cross laminated plywood samples. Ignition time data was found to exhibit significant positive skew and 20-30 trials were required for the reduction in uncertainty with each additional trial to fall below 0.1s at each heat flux. The normalized uncertainty in ignition time was greatest at 50 kW/m2 and was 20-70% than at 20 kW/m2 and 40 kW/m2. Significant variability was observed in the extinction characteristics of samples exposed to 40 kW/m2 where 39 samples experienced self-extinction while the remainder sustained combustion until burnout. Uncertainty in MLR at extinction for these trials was nearly double that of trials exposed to 20 kW/m2. These results exhibit the significance of large trial quantities when determining flammability characteristics.

<p>In an effort to better understand the physical mechanisms responsible for pool boiling heat transfer, an analytical model is developed that better describes the changing shape and size of a growing bubble. Indeed, any analysis of thermal transport due to nucleate pool boiling requires bubble frequency predictions which are intimately linked to bubble volume. The model is developed and validated for quasi-static bubble growth due to gas injection and for bubble growth due to vaporization within the heat-transfer controlled growth regime; it highlights the need to include the asymmetric nature of growing bubbles when modeling bubble growth.</p> <p>In addition, a numerical study of quasi-static bubble shape for both adiabatic bubble growth and vapour bubble growth provides insight into the dependence the bubble shape evolution has on the Bond number. In so doing, bubble profiles generated from a numerical treatment of the Capillary equation are benchmarked to quasi-static gas injected bubble formations and to heat-transfer controlled vapour bubble formations.</p> <p>The numerical treatment of bubble shape evolution leads to a simplifying bubble geometry for low Bond number applications. The geometric model accounts for bubble shape transformation throughout the bubble growth cycle including the necking phenomenon. An analytical model of quasi-static adiabatic bubble growth is accordingly developed based on the proposed low Bond number geometric model; it is coupled with a geometric detachment relation and a force balance detachment criterion that are dependent on the Bond number. The resulting predicted bubble growth characteristics, such as profile, volume, centre of gravity and aspect ratio, are validated with the benchmarked numerical treatment of the problem.</p> <p>Furthermore, the low Bond number geometric model is applied to bubble growth due to vaporization. In order to solve the mass-energy balance at the vapour bubble interface, a spherical surface area is commonly assumed. This leads to the need for correction factors and provides little insight into the physical mechanism responsible for bubble shape. In this study, the transitioning shape of a vapour bubble is considered in the integral analysis of the interfacial mass-energy balance. The model predicts the following bubble growth characteristics: profile, volume, centre of gravity, and aspect ratio.</p> / Doctor of Philosophy (PhD)

<p>With the rising prices of energy and the harmful environmental effects many of conventional energy generation techniques the world is pushing for new, cleaner, more efficient and more environmental renewable energy sources. Thermoelectric generators are one of the potential solutions to these problems of unclean and expensive energy. Thermoelectric generators are solid state devices that convert thermal energy into useful electrical energy.</p> <p>Over the last ten years the progress in materials science have led to advancements in thermoelectrics. However as of yet no standardised method of testing thermoelectric generators has been established and as such data provided for thermoelectric generators is regarded as questionable. This thesis deals with two commercial thermoelectric generator models, TEG1 12610-5.1 AND TEG1B 12610-5.1, and quantifies the deviation of the manufacturer’s specifications to what is experimentally achieved by the generators as 147% and 22% respectively. The variance of the outputs between thermoelectric generators was measured by comparing the maximum power output for the models in question over a sample size of four, it was found to be as much as 20% and 8% respectively.</p> <p>A full characterisation of the thermoelectric generators is performed on the two generator models to obtain the data as to their power output and thermal conductivity for the purpose of design of a waste energy harvesting device. The full characterisation was also used to validate the testing apparatus as a device capable for the use as a standardised method of characterising the performance of thermoelectric generation modules.</p> <p>A mechanistic model is created using the experimental characterisation data. This mechanistic model has the ability to accurately predict the voltage and current output of the thermoelectric generator models under any given temperatures and electrical loading condition with a minimum R-squared value of 0.94. The thermal conductivity is also found to be predictable using an established equation modified with an empirical constant.</p> / Master of Applied Science (MASc)

The emergence of metal Additive Manufacturing (AM) over the last two decades has opened venues to mitigate the challenges associated with stochastic open-cell metal foams manufactured through the traditional foaming process. Regular lattices with user-defined unit cell topologies have been reported to exhibit better mechanical properties in comparison to metal foams which extend their applicability to multifunctional heat exchangers subjected to both thermal and mechanical loads. The current study aims at investigating the thermal-hydraulic characteristics of promising novel unit cell topologies realizable through AM technologies. Experimental investigation was conducted on four different topologies, viz (a) Octet, (b) Face-diagonal (FD) cube, (c) Tetrakaidecahedron, and (d) Cube, printed in single-cell thick sandwich type configuration in 420 stainless steel via Binder Jetting technology at same intended porosity. The effective thermal conductivity of the samples was found to be strongly dependent on the lattice porosity, however, no significant dependence on the unit-cell topology was demonstrated. Face-diagonal cube lattice exhibited the highest heat transfer coefficient and pressure drop, and consequently provided the lowest thermal-hydraulic performance. A procedure to incorporate the manufacturing-induced random roughness effects in the samples during numerical modelling is introduced. The numerical simulations were conducted on samples exhibiting the roughness profiles having statistically same mean roughness as the additively manufactured coupons and the results were compared to that obtained from the intended smooth-profiled CAD models that were fed into the printing machines. The analysis showed that inclusion of roughness effects in computational models can significantly improve the thermal performance predictions. Through this study, we demonstrate that additively manufactured ordered lattices exhibit superior thermal transport characteristics and future developmental efforts would require extensive experimentations to characterize their thermal and flow performance as well as local surface quality and AM-induced defect recognition. Experimental findings would also need to be supported by computational efforts where configurations which closely mimic the real AM parts could be modeled. A combined experimental-numerical framework is recommended for advancements in metal additive manufacturing-enabled enhanced heat transfer concepts.