For heat transfer through building insulations such as fiberglass, radiation and conduction are important modes of heat transfer. Moreover, materials like fiberglass scatter radiation in a highly anisotropic manner. The equations for heat transfer by simultaneous conduction and radiation are a coupled pair, one of which is of the nonlinear integrodifferential type. Exact solution for transient heat transfer in this case is not available, and the approximate solution available is the two-flux model. The two-flux model does not give good results for transient, combined mode heat transfer, through an absorbing, emitting, and anisotropically scattering medium. In this thesis a higher order approximate solution has been developed. It is found that this model gives appreciably better results than the two-flux model. / Master of Science

The effects of a shock wave passing through a blade passage on surface heat transfer to turbine blades were measured experimentally. The experiments were performed in a transonic linear cascade which matched engine Reynolds number, Mach number, and shock strength. Unsteady heat flux measurements were made with Heat Flux Microsensors on both the pressure and suction surfaces of a single blade passage. Unsteady static pressure measurements were made using Kulite pressure transducers on the blade surface and end walls of the cascade. The experiments were conducted in a stationary linear cascade of blades with heated transonic air flow using a shock tube to introduce shock waves into the cascade. A time-resolved model based on conduction in the gas was found to accurately predict heat transfer due to shock heating measured during experimental tests without flow. The model under-predicted the experimental results with flow, however, by a factor of three. The heat transfer increase resulting from shock passing in heated flow averaged over 200 its (typical blade passing period) was found to be a maximum of 60% on the pressure surface near the leading edge. Based on experimental results at different flow temperatures, it was determined that shock heating has the primary effect on heat transfer, while heat transfer increase due to boundary layer disturbance is small. / Master of Science

Economic viability of fish production in recirculating aquaculture system (RAS) facility depends on minimizing the energy requirements of operating such facilities. The fish growth and water quality aspects of RAS have been studied in considerable details. However, the understanding of the thermal environment of RAS lags behind. A step-wise steady-state thermal model was developed to simulate the daily heating, ventilation, water pumping, biofilter operation, and lighting energy requirements over a production cycle. The model was validated using temperature and energy data collected from RAS facility of Virginia Tech during 1992. Model simulations were performed with various production scenarios. The energy cost of fish production ($/kg) was used to evaluate different scenarios with and without heat recovery from discharged system water. Building heating required the most (40 % - 70 % of total) energy followed by water pumping, biofilter operation, lighting, and ventilation. Water replacement was the most dominant factor in determining the facility’s heating energy requirement. Heat recovery from discharged system water indicated significant drop (up to 40 %) in energy cost of fish production. / Master of Science

Gas turbine blade tip heat transfer and tip gap flow phenomena has been explored experimentally in a stationary cascade for blade exit Mach numbers = 1.2 to 1.4. Experimental results were found to agree well with qualitative predictions performed at GE Aircraft Engines. The pressure distribution in the blade tip cavity of a grooved tip blade was found to vary little with either Mach number or tip gap height. The tip cavity pressure was, however, a strong function of location. The tip cavity pressure distribution coupled with the pressure side distribution near the tip was speculated to drive the leakage flow across the blade tip from mid-chord aft based on surface flow visualization studies using an oil/dye mixture. Heat flux on the tip cavity floor was successfully measured using a thin-film Heat Flux Microsensor. Results of these measurements are consistent with previous studies in the subsonic regime. The convection coefficients on the tip cavity floor were found to be three times those found on the suction side airfoil surface near the trailing edge. Convection coefficients were found not to vary with either tip gap height or Mach number. The fluctuating component of heat flux was found to be at least 25% of the total heat flux. / Master of Science

The electric field-induced reduction of heat transfer from a rod-stabilized diffusion flame and a step-stabilized premixed flame was investigated. The fuel examined was propane. Inlet velocity for the diffusion mode was a nominal value of 3.4 m/s with nominal air/fuel ratios of 420, 320, and 270. Inlet velocities ranged from 4.5 to 9.9 m/s for the premixed mode with equivalence ratios of 0.65 to 1.03. Maximum applied voltages for the diffusion and premixed modes were 8.0 and 6.6 kVDC, respectively. The field was applied in a direction perpendicular to the flow. Heat transfer amelioration was quantified using records of temperature versus downstream distance from the stabilizer acquired for the external surface of the heatloaded electrode which was exposed to the ambient environment. In addition, shadowgraphs and photographs were used to observe any alteration of flame position or of the bulk flowfield. These observations were used to investigate mechanisms potentially responsible for heat transfer reduction. The rod-stabilized diffusion mode displayed some field-induced reduction in heat transfer. Both bulk flow alteration and reduction in radiation (associated with soot) were concluded to be responsible. Flame impingement on the heat-loaded electrode was reduced by a field-induced increase in flow along the surface. Flame luminosity was reduced by the electric field (presumably due to a field-induced modification of soot production and/or destruction). This caused a reduction in radiative heat transfer. No heat-transfer amelioration was noted for the premixed step-stabilized mode. This was attributed primarily to a geometry not accommodating to field-induced heat transfer reduction. Higher velocities and a lower presence of soot than the diffusion mode and problems associated with flame impingement on both electrodes (reduces maximum voltages and distorts field), also contributed to the negative result. Limited displacement of the luminous portion of the reaction zone was noted. / Master of Science

The concept of using heat flux measurements to provide the input for a feedforward temperature control loop is investigated. The feedforward loop is added to proportional and integral feedback control to increase the speed of the response to a disturbance. Comparison is made between the feedback and the feedback plus feedforward control laws. The control law with the feedforward control loop is also compared to the conventional approach of adding derivative control to speed up the system response to a disturbance. The concept was tested using a simple flat plate heated on one side and exposed to a step change in the convective heat loss on the other side. A controller was constructed using an analog computer to compare the feedforward and feedback approaches. The conventional control approach was tested using a commercial temperature controller. The feedback and feedforward approaches were also simulated. The results showed that the feedforward control approach produced significant improvements in the response to the disturbance. The integral of the squared error between the setpoint and actual temperature was reduced by approximately 90 percent by the addition of feedforward control to the feedback control. The maximum temperature deviation from the setpoint was also reduced by 70 percent with the addition of feedforward control. Qualitative agreement was obtained between the experimental results and the computer simulations. The conventional approach of adding derivative control to the proportional and integral control showed an increase of 20 percent in the integral of the squared error, but offered no significant improvement in the maximum temperature deviation. The addition of derivative control also caused the stability of the system to decrease, while the addition of feedforward had no adverse effects on the system stability. The concept of using heat flux measurements for feedforward control was successfully demonstrated by both simulations and experiments. / Master of Science

An investigation was conducted to determine the effect of organized flow pulsations on mean heat transfer from a single cylinder, in-line arrangements and perpendicular arrangements of cylinders. Pulsation frequencies of up to twice the natural vortex shedding frequency and zero to peak. amplitudes as high as 36 percent were used. Pulsations were sinusoidal with at least 93 percent of the power at the fundamental frequency. Turbulence levels (Tu=0.5 percent) were not altered by the addition of unsteady flow. Reynolds number ranged from 23,000<Re<49,000. Results for heat transfer on the front and back of the cylinder are given for a constant wall temperature boundary condition. Heat transfer measurements were made by applying a heat balance to a thick walled copper tube divided into four individually heated segments with guard. heaters located at each end. Mean heat transfer was found to increase for all three arrangements when organized flow pulsations were applied. For a single cylinder and for perpendicular arrangements, heat transfer increases were found primarily on the back of the cylinder. For in-line arrangements, increases occurred mostly on the front of the cylinder. for the range of pitch ratio most useful to heat exchanger design, in-line arrangements were found to have a higher Nusselt number than perpendicular arrangements. / Master of Science

A theoretical study of convective heat transfer is presented for a laminar flow subjected to an axial variation in the external heat transfer coefficient (or dimensionless Biot number). Since conventional techniques fail for a variable boundary condition parameter, a variable eigenfunction approach is developed. An analysis is carried out for a periodic heat transfer coefficient, which serves as a model for heat transfer from a duct fitted with an array of evenly spaced fins. Three solution methods for the variable eigenfunction technique are examined: an Nth order approximation method, an iterative method and a stepwise periodic method. The stepwise periodic method provides the most convenient and accurate solution for a stepwise periodic Biot number. Graphical results match exactly to ones obtained by Charmchi and Sparrow from a finite-difference scheme. A connected region technique is also developed to provide limited exact results to test the validity of the three solution methods. The study of a finned duct by a stepwise periodic Biot number is carried out via a parametric study, an average (constant) Biot number approximation and an assumed velocity profile analysis. Results for the parametric study show that external finning yields substantial heat transfer enhancement over an unfinned duct, especially when the Biot number of the unfinned regions is low. A decrease in the interfin spacing causes increased enhancement. Variations of the period of the Biot number causes relatively small changes in enhancement as long as the ratio of finned to unfinned surface remains unchanged. An average (constant) Biot number approximation for a specified finned tube is compared to the stepwise periodic Biot number solution. The results show that the constant Biot number approximation provides accurate results. Finally, the results for the influence of the assumed velocity profile demonstrate that a constant velocity flow provides increased heat transfer and more effective enhancement by external finning than a laminar fully developed flow, especially at high Biot numbers. This study provides insight into heat transfer enhancement due to finning and also develops a solution methodology for problems involving variable boundary condition parameters. / M.S.

The continuous miniaturization of electronic devices and the corresponding increase in computing powers have led to a significant growth in the density of heat dissipation within these devices. This increase in heat generation has challenged conventional air fan cooling and alternative solutions for heat removal are required to avoid overheating and part damage. Micro/Mini channel heat sinks (M/MCHS) that use liquids for heat removal appear as an attractive solution to this problem as they provide large heat transfer area per volume. Mini/microchannels traditionally have suffered from geometrical and material restrictions due to fabrication constraints. An emerging new additive manufacturing technique called binder jetting has the potential to overcome some of those restrictions. In this study, a straight minichannel heat sink is manufactured from stainless steel using binder jetting, and it is experimentally evaluated. The hydraulic performance of the heat sink is tested over a range of Reynolds numbers (150-1200). The comparison between the hydraulic results and standard correlations confirms that the targeted geometry was produced, although the high surface roughness created an early transition from laminar-to-turbulent flow. The heat transfer performance was also experimentally characterized at different heat flux conditions ($3000W/m^2$, $5000W/m^2$, $6500W/m^2$), and a range of Reynolds numbers (150-800). These results indicated that convection heat transfer coefficients on the order of $1000 W/m^2-K$ can be obtained with a simple heat sink design. Finally, the effects of the contact resistance on the results are studied, and contact resistance is shown to have critical importance on the thermal measurements. / Master of Science / The continuous miniaturization of electronic devices and the corresponding increase in computing powers have led to a significant growth in the density of heat dissipation within these devices. This increase in heat generation has challenged conventional air fan cooling and alternative solutions for heat removal are required to avoid overheating and part damage. Micro/Mini channel heat sinks (M/MCHS) that use water instead of air for heat removal appear as an attractive solution to this problem as they provide large heat transfer area per volume due to the small channels. Mini/microchannels are distinguished from conventional channels by the hydraulic diameter, where they range from $10mu m$ to $2mm$. M/MCHS are typically manufactured from a highly conductive metals with the channels fabricated on the surface. However, mini/microchannels traditionally have suffered from geometrical and material restrictions due to fabrication constraints. Complex features like curves or internall channels are difficult or even impossible to manufacture. An emerging new additive manufacturing technique called binder jetting has the potential to overcome some of those restrictions. Binder jetting possess unique advantageous as it uses precise control of a liquid binder applied to a bed of fine powder to create complex geometries Furthermore, it does not require extreme heating during the fabrication process. The advantages of binder jetting include that it is low cost, high speed, can be applied to a variety of materials, and the ability to scale easily in size. In this study, a straight minichannel heat sink is manufactured from stainless steel using binder jetting, and this heat sink is experimentally evaluated. The hydraulic performance of the heat sink is tested over different water flow rates (Reynolds numbers between 150-1200). The comparison between the hydraulic results and standard correlations confirms that the targeted geometry was produced, although the high surface roughness created an early transition from laminar-to-turbulent flow. The surface roughness effect should be considered in future designs of additively manufactured minichannels. The heat transfer performance was also experimentally characterized at different heat flux conditions ($3000W/m^2$, $5000W/m^2$, $6500W/m^2$), and different water flow conditions (Reynolds numbers 150-800). These results indicated that convection heat transfer coefficients on the order of $1000 W/m^2-K$ can be obtained with a simple heat sink design. However, a mismatch between the experimental data and the correlation requires further investigation. Finally, the effects of the contact resistance on the results are studied, and contact resistance is shown to have critical importance on the thermal measurements.

A local heat-flux measurement system was built, calibrated and tested for use in unsteady flows. The system was designed to maintain constant wall temperature boundary conditions. The measuring element is a thin-film heat flux gage made by sputter-coating gold on a substrate. A constant-temperature anemometer is used to maintain the thin-film gage at a specified temperature under fluctuating conditions. A separate temperature control system maintains the surrounding boundary at the gage temperature. The system was calibrated for both steady and unsteady flows using a specially designed calibrator for local heat flux gages. The steady calibration was done with predominantly convective heat transfer . The unsteady calibration was achieved by adding oscillating radiant energy to the surface. Consequently, quantitative results can be obtained for both mean and fluctuating components of the heat transfer. The frequency response was good to 92 hertz. Sample results are presented for unsteady heat transfer caused by the vortex shedding from a cylinder in a steady crossflow. The shedding frequency was 82 hertz. / M.S.