B.B. Mikic

THERMAL CONTACT CONDUCTANCE; THEORETICAL CONSIDERATIONS

Abstract Thermal contact conductance of nominally flat surfaces in contact was considered. The emphasis of the work is on effect of the mode of deformation on the value of conductance. Explicit expressions for thermal conductance were derived for cases of: (1) Pure plastic deformation (2) plastic deformation of the asperities and elastic deformation of the substrate and (3) pure elastic deformation. The last two important cases are considered for the first time here. Criteria which determine mode of deformation is also presented.

On bubble growth rates

A simple general relation for bubble growth rates in a uniformly superheated liquid was derived. The relation is valid in both regions: inertia controlled and heat diffusion controlled growth, respectively. The derived relation is compared with the existing experimental results for bubble growth in a uniformly superheated liquid with very good agreement. The results are further extended to the bubble growth in a non-uniform temperature field which approximates the conditions present in a nucleate boiling from a heated surface.Abstract A simple general relation for bubble growth rates in a uniformly superheated liquid was derived. The relation is valid in both regions: inertia controlled and heat diffusion controlled growth, respectively. The derived relation is compared with the existing experimental results for bubble growth in a uniformly superheated liquid with very good agreement. The results are further extended to the bubble growth in a non-uniform temperature field which approximates the conditions present in a nucleate boiling from a heated surface.

On mechanism of dropwise condensation

Abstract The effect of surface thermal properties on dropwise condensation is considered. The physical explanation for this effect is given by considering constriction phenomena in metalic surfaces caused by a non-uniform heat flux distribution over the condensing surfaces. From available experimental data it is shown that surface thermal properties are one of the controlling factors in the dropwise condensation. It is found that for stainless steel as the condensing surface about 84 per cent of the total resistance was the contribution of the resistance occurring in the plate itself due to heat flux non-uniformity over the surface: for copper this contribution was 20 per cent.

Numerical and experimental studies of self-sustained oscillatory flows in communicating channels

Abstract A combined numerical and experimental investigation of flow fields and thermal phenomena in communicating channels is performed to gain insight into the operation of compact heat exchange surfaces with interrupted plates. The geometric parameters are selected to excite and sustain the normally damped Tollmien-Schlichting modes. As a result, traveling waves are observed at relatively low Reynolds numbers, inducing self-sustained oscillatory flows that significantly enhance mixing. The critical Reynolds number at which oscillations are first observed in the periodic, fully developed flow region is determined. The numerical results are obtained by direct numerical simulation of the time-dependent energy and Navier-Stokes equations using a spectral element-Fourier method. The oscillatory heat transfer phenomenon is visualized experimentally using real-time, holographic interferometry. For periodic, fully developed flow conditions, the temperature fields are recorded utilizing high-speed cinematography. The experimental visualizations of the thermal waves verify the numerical predictions of the thermal-fluid structure and evolution of communicating-channels flows.

Exploiting hydrodynamic instabilities. Resonant heat transfer enhancement

Abstract We introduce here the concept of resonant heat transfer enhancement based on excitation of shear-layer instabilities present in internal separated flows. Exploitation of natural instabilities requires: creation of a system with separated flow; determination of the systems resonant frequency; and excitation of that frequency with appropriate modulation. The resulting large scale motions lead to significant lateral mixing and correspondingly dramatic heat transfer enhancement. The method is applicable both in laminar and turbulent flows. Results, experimental and numerical, for a subcritieal grooved channel flow and a cross flow around a cylinder are presented. For the case of grooved channel, up to three-fold enhancement of heat transfer is observed when the flow is modulated at the systems natural frequency.

Thermal stresses in frozen organs

Abstract An analysis was performed to determine the thermal stresses in the solid region of an organ frozen so that a constant cooling rate is imposed on its outer surface. The analysis shows that at the instant the water freezes at a certain location in the organ, compressive azimuthal stresses develop in the region close to the change of phase front. The compressive asimuthal stresses decrease and become tensile at that location as the change of phase front propagates further. The radial stresses are always compressive. It is hypothesized that those stresses might induce mechanical damage to the cellular components of the organ. The analysis shows that the magnitude of these stresses is a function of the material properties and the product of the outer surface cooling rate and the square of the outer surface radius.

Thermal constriction resistance due to non-uniform surface conditions; contact resistance at non-uniform interface pressure

Thermal constriction resistance due to nonuniform metal surface conditions, considering macroscopic contact resistance for nonuniform interface pressure distribution

Minimum-dissipation heat removal by scale-matched flow destabilization

Abstract Minimum-dissipation convective heat removal from a wall to a flowing fluid stream by scale-matched flow destabilization is considered. A universal scaling for the optimization problem is developed, and it is shown from Reynolds analogy-hydrodynamic stability arguments that the dissipation-minimizing transport solution corresponds to flow destabilization at a Reynolds number (and associated spatial scale) that increases (decreases) with increasing thermal load. The significant dissipation savings possible through optimization are demonstrated in a sample optimization study for heat transfer in a channel based on experimental data for several enhancement procedures. The optimizing transport enhancement scheme is shown to proceed from macroscale eddy promoters to microgroove roughness elements with increasing thermal load, thus verifying the validity of the scale-matched destabilization theory.

An experimental investigation into the effect of surface thermal conductivity on the rate of heat transfer in dropwise condensation

Abstract The magnitude of the surface thermal conductivity effect in dropwise condensation heat transfer is determined experimentally in this work. The heat-transfer coefficient for a low conductivity surface (stainless steel) was measured using deposited thin-film resistance thermometers. Copper-surface data was obtained using a conventional test section. The results are in agreement with a previously developed analytical model.

An analysis of the effect of surface thermal conductivity on the rate of heat transfer in dropwise condensation

Abstract An analysis of the effect of the condenser material thermal properties on the dropwise condensation heat-transfer coefficient is reported here. The synthesis of single-droplet constriction resistance solutions with the known droplet distribution leads to a simple correlation for the effect. The correlation agrees well with the bulk of existing experimental data for water; other fluids may be treated via the analytical path developed.