The paper explores the use of open-celled metal foams as compact heat exchangers, exploiting convective cooling. An analytical model is developed for model foams with simple cubic unit cells consist-ing of heated slender cylinders, based on existing heat transfer data on convective crossflow through cylin-der banks. A foam-filled channel having constant wall temperatures is analyzed to obtain the temperature distribution inside the channel as a function of foam density, cell size and other pertinent heat transfer par-ameters. Two characteristic length scales of importance to the problem are discussed: the minimum channel length required for heating the fluid to its goal temperature and the thermal entry length beyond which the transfer of heat between fluid and channel wall assumes a constant coecient. The overall heat transfer coecient of the heat exchanging system is calculated, and the pressure drop experienced by the fluid flow obtained. These results are used to analyze and guide the design of optimum foam structures that would maximize heat transfer per unit pumping power. Two examples are given to demonstrate the applicability of the analytical model: heat sinks for high power electronic devices and multi-layered heat exchangers for aeronautical applications. The present model perhaps oversimpli

This paper presents an analytical and numerical study on the heat transfer characteristics of forced convection across a microchannel heat sink. Two analytical approaches are used: the porous medium model and the fin approach. In the porous medium approach, the modified Darcy equation for the fluid and the two-equation model for heat transfer between the solid and fluid phases are employed. Firstly, the effects of channel aspect ratio (χs) and effective thermal conductivity ratio ( ) on the overall Nusselt number of the heat sink are studied in detail. The predictions from the two approaches both show that the overall Nusselt number (Nu) increases as χs is increased and decreases with increasing . However, the results also reveal that there exists significant difference between the two proaches for both the temperature distributions and overall Nusselt numbers, and the discrepancy becomes larger as either as or is increased. It is suggested that this discrepancy can be attributed to the indispensable assumption of uniform fluid temperature in the direction normal to the coolant flow invoked in the fin approach. The effect of porosity (e) on the thermal performance of the microchannel is subsequently examined. It is found that whereas the porous medium model predicts the existence of an optimal porosity for the microchannel heat sink, the fin approach predicts that the heat transfer capability of the heat sink increases monotonically with the porosity. The effect of turbulent heat transfer within the microchannel is next studied, and it is found that turbulent heat transfer results in a decreased optimal porosity in comparison with that for the laminar flow. A new concept of microchannel cooling in combination with microheat pipes is proposed, and the enhancement in heat transfer due to the heat pipes is estimated. Finally, two-dimensional numerical calculations are conducted for both constant heat flux and constant wall temperature conditions to check the accuracy of analytical solutions and to examine the effect of different boundary conditions on the overall heat transfer.

The fluid-flow and heat-transfer features of cellular metal lattice structures made from copper by transient liquid phase (TLP) bonding and brazing of plane weave copper meshes (screens) were experimentally characterized under steady-state forced air convection. Due to the inherent structural anisotropy of this metal textile derived structure, the characterizations were performed for several configurations to identify the preferable orientation for maximizing thermal performance as a heat dissipation medium. Results show that the friction factor of bonded wire screens is not simply a function of porosity as stochastic materials such as open-celled metal foams and packed beds, but also a function of orientation (open area ratio). The overall heat transfer depends on porosity and surface area density, but only weakly on orientation. Comparisons with stochastic metal foams and other heat dissipation media such as packed beds, louvered fins and microtruss lattice cellular materials suggest that wire-screen meshes compete favorably with the best available heat dissipation media. The overall thermal efficiency index of the copper textiles-based media is approximately three times larger than that of stochastic copper foams, principally because of the lower pressure drop encountered during coolant propagation through the periodic wire-screen structure.

The propagation of a pre-existing edge crack across a finite plate subjected to cold shock has been studied. The plate, initially at uniform temperature, is exposed to a cold shock on one surface whilst three dierent types of heat transfer boundary condition are separately considered for the opposing face: cold shock, thermal insulation and fixed temperature. For all three boundary conditions, the plate experiences tensile stress near the cold-shocked surface and compressive stressing near the mid-plane. Consequently, a Mode I edge crack extending into the compressive region may grow in one of three different modes: continued extension in plane strain, channelling and spalling. The thermal shock conditions governing each failure mode are quantified, with a focus on crack channelling and spalling. The dislocation method is employed to calculate the energy release rates for plane strain cracking and steady-state channelling. For steady-state spalling, the energy release rate is obtained by an energy analysis of elastic beams far ahead and far behind the crack tip. Analytical solutions are also obtained in the short crack limit in which the problem is reduced to an edge crack extending in a half space; and the parameter range over which the short crack solution is valid for a finite plate is determined. Failure maps for the various cracking patterns are constructed in terms of the critical temperature jump and Biot number, and merit indices are identified for materials selection against failure by thermal shock.

A minimum-weight flexural actuator is designed. The actuator comprises a triangular corrugated core with shape memory alloy (SMA) faces. It is clamped at one end and free at the other. For design and optimization, the temperature history of the face sheets upon heating and subsequent cooling is 7rst obtained as a function of the cooling e8ciency (Biot number) and the operational frequency deduced. Based upon this response, a phenomenological model is employed to represent the martensite evolution. Thereafter, the end de'ection is calculated as a function of temperature. The minimum weight is calculated subject to the provisos that: (i) the end de'ection attains a speci7ed value; (ii) the power consumed is less than the upper limit of the supply; and failure is averted by (iii) face=core yielding and (iv) face=core buckling; (v) the operational frequency of the panel achieves a speci7ed limit.