The subject to be discussed in this thesis is an experimental problem in heat transfer across a carbon-liquid helium I interface. The interest in this particular problem arose from experimental work first presented by Luce (1) concerning a similar heat transfer problem from a heated bismuth surface immersed in liquid helium. This study produced some interesting results which it was hoped could be duplicated with carbon. The choice of carbon as an alternative material was based on reasons which will be discussed later. Before beginning a detailed discussion of the problem a history of its development and the results which were obtained using bismuth single crystals will first be presented.

The application of thermoacoustic phenomena for cooling purposes has a comparatively short history. However, recent experiments have shown that thermoacoustic refrigeration can achieve practical significance for both every day cooling in households and cryocooling for scientific purposes due to its high reliability, environmental safety and functioning under extreme conditions. We build a thermoacoustic refrigerator driven by a commercial loudspeaker. It was equipped with a vacuum pump and an entrance port for introducing different gases under different pressures as working fluids. It contained two thermocouples and a pressure transducer for quantitative measurements of the basic performance. The resonance frequency of the tube for different gases has been determined and compared to the theoretical value. The temperatures of the hot and the cold heat exchanger have been measured. Also, a simple thermoacoustic oscillator for demonstration purposes was built. After immersing one end in liquid nitrogen or heating up the other end with a bunsen burner it started to oscillate and emit a sound.

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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.

Graduation date: 1975

A unique channel surface enhancement technique via diffusion-bonding a layer of conductive fine wire mesh onto the heating wall was developed and used to experimentally study flow boiling enhancement in parallel microchannels. Each channel was 1000 μm wide and 510 μm high. A dielectric working fluid, HFE 7000, was used during the study. Two fine meshes as well as two mesh materials were investigated and compared. According to the flow boiling curves for each channel, the amount of wall superheat was greatly reduced for all the mesh channels at four stream-wise locations; and the critical heat fluxes (CHF) for mesh channels were significantly higher than that for a bare channel in the low vapor quality region. According to the plots of local flow boiling heat transfer coefficient h versus vapor quality, a consistent increasing trend for h with vapor quality was observed for all the tested channels until the vapor quality reached approximately 0.4. However, the three mesh channels showed much higher values of h than the bare channel, with the 100 mesh copper performing the best. Visualization using a high-speed camera was performed thereafter to provide some insights to this enhancement mechanism. A significant increase in nucleation sites and bubble generation was observed, and departure rates inside the mesh channels were attributed to the flow boiling enhancement. A sudden increase of h for mesh channels can also be attributed to the characteristics of nucleate boiling and indicates that nucleate boiling was the dominant heat transfer mode. Another interesting point observed was that the 100 mesh bronze outperformed the 200 mesh bronze for most of the studies. This suggests that nucleations happened inside the mesh openings, instead of on the mesh openings. In addition, an optimal mesh size should exist for HFE 7000 flow boiling. / Graduation date: 2006

Graduation date: 2001

Production of surimi represents a potential utilization of a number of low-valued fish species, one of which, Pacific whiting, represents the largest biomass off the West Coast of the United States. However, a protease enzyme softens the fish flesh in Pacific whiting and limits the expansion of surimi production. Many studies have demonstrated the importance of time and temperature in minimizing the texture softening. An optimal design of the surimi seafood process is possible only when an accurate prediction of the time-varying temperature distribution throughout the surimi product can be obtained. This provides a measure of the heating rate and the extent of thermal processing. Such a prediction necessitates a study of the surface heat transfer coefficient which is one of the most important parameters for the heat transfer analysis. Associated with automated-machinery processing of surimi seafoods, a full understanding of the heat transfer coefficient (h) is especially important because high-quality surimi products using Pacific whiting only can be obtained through rapid and controlled heating. This study was intended to determine transient surface heat transfer coefficients in a steam heating environment, simulating the widely-used steam heating of thin-sheet surimi paste in the seafood industry. In determining the heat transfer coefficient, many different methods have been used including the inverse calculation method, the lumped mass method and the heat flux method. This study employed all three to measure and model the heat transfer coefficient (h) under similar steam conditions. A comparative evaluation was made to define the best method and model for the h determination. The inverse calculation method produced an h model which, when applied to a heat transfer analysis, provided the best agreement between predicted and experimental temperature profiles at three locations in surimi paste during a 1000-sec cooking period. The lumped mass method overestimated the heat transfer coefficients to food; the heat flux method gave inconsistent measurements. It is a classic inverse problem to estimate surface heat transfer coefficients from temperature measurements inside a product, a procedure which involves solution of the inverse heat conduction problem and parameter optimization. A whole domain function specification procedure was developed for the inverse calculation method. This procedure simulates heat transfer coefficients as specified functions by estimating all the unknown parameters in the functions over the total time interval. A nonlinear regression computer program was written for the inverse calculation of surface heat transfer coefficients, incorporating the implict Crank-Nicolson scheme for the finite-difference formulation of the one-dimensional heat conduction problem and the downhill simplex method for parameter optimization. This inverse calculation method provided relatively accurate models of the surface heat transfer coefficient. / Graduation date: 1997

University of Macau / Faculty of Science and Technology / Department of Electromechanical Engineering