Experimental investigation of 3D woven Cu lattices for heat exchanger applications

Zhao et al. (2016) studied the performance of the Copper (Cu) woven lattices in both standard and topology optimized architecture using three flow patterns (axial, focused bifurcated and full bifurcated) and two working coolants (water and air). The work on 3D woven Cu lattices for heat exchangers appeared recently in International Journal of Heat and Mass Transfer.

Cellular metals are metallic bodies made of pore structures in which fluid or gas are dispersed. They possess unique combinations in fluidic, mechanical, thermal property or energy absorption. Common example is Heat Exchangers, commonly found in various applications. Heat exchangers are multifunctional; they offer various functions such as heat transfer enhancement, structural support and good damping properties.

Metal forms are the most common stochastic heat exchangers due to their low density, low cost and unique mechanical and electrical properties. However, they have various limitations due to their high impedance, large pressure loss, deformation under mechanical loading resulting to minimal stiffness and strength coupled with their irregular cell geometries.

Periodic cellular structure, another form of cellular heat exchanger has been developed recently. They have been proposed to function efficiently from simple structures to more complex structures due to their periodicity and low impedance. However improvement can be done to broaden its multi-functionality.

Report has been done previously (Zhao et al. Acta Materialia 81, 326–336, 2014) on two 3D woven Cu lattice architectures; first is called “standard” which is densely packed woven architecture while the other is called “optimized” which is topology-optimized.

A 3D woven and brazed Cu lattices that measured approximately 3mm in thickness (Z), 35mm in width (Y) and millimeters to meter in length (X) was fabricated. Coolant flows and heat transfer varies significantly with orientation as Z-direction has 3 times smaller permeability than the X and Y-directions. Heat transfer was focused in Y-direction as it had the highest density of straight Cu wires that span the full width of the block. For cooling, axial (1-D) flow was generated in X-direction with the highest permeability and bifurcated (2-D) flow in X and Y directions. Bifurcated flow was studied either by full bifurcated case of the whole 76.2 mm x 23.4 mm across cross-section inlet or focused bifurcated case that restricts inlet to only central 25.4 mm x 23.4 mm.

Results on pressure drop taken in each woven block in different flow patterns showed twice as large across the standard architecture when compared to optimized architecture for same flow rates. This can be due to higher volume fraction of materials in standard architecture (51.1%) compared to optimized architecture (39.4%) and pore structure of optimized architecture designed using topology optimization to maximize flow in X-direction. Pressure drop was seen to be the highest in axial flow pattern followed by focused bifurcated and full bifurcated respectively due to shorter average travel distance and large cross-sectional area within full bifurcated patten.

Friction factors were calculated for the three flow patterns and both coolants. It was seen that all measured friction factors decrease as Reynolds number increase but friction factors is weakest for full bifurcated flow pattern, absolute values of friction factor are slightly lower for optimized architectures compared to standard architecture, friction factors diverge for water and air in both architectures as one moves toward lower Reynolds number and flow resistance ascends from full bifurcated to focused bifurcated to the axial flow pattern.

Average Surface Temperature (T_S) and temperature variation across the surface (T) were found to be lower in standard weaves when compared to optimized weaves. This is due to more wires and surface area for effective thermal conduction and convection between solid Cu and working coolant coupled with smaller pore sizes. Axial flow possessed the lowest T_S while full bifurcated flow has the highest T_S. It was also discovered that T first rises then falls with increasing flow rates for air test which is not observed in water test due to air having lower heat capacity and density.

With the following results, standard weave in an axial flow pattern may be the best for dissipating heat while an optimized weave in one of the two bifurcated patterns are better for applications where pumping power and mass is limited or temperature uniformity is required.

Bifurcated flow patterns are superior in general to axial patterns due to lower pressure drop and T in the former.  Zhao et al. (2016), 3D woven Cu lattices when compared to the other heat dissipating media has various advantages in higher flow resistance and heat transfer of the weaves coupled with higher thermal efficiency. Its feature combined with bifurcated flow offers excellent heat transfer and damping properties.