Sean W. Reilly

Improving Biporous Heat Transfer by Addition of Monoporous Interface Layer

Biporous evaporator wicks, generated by sintering copper particles into semi-uniform clusters, were demonstrated to achieve high flux, heat transfer performance for use in heat pipes by Semenic (2007). The effective thermal conductivity of thick biporous wicks at high heat fluxes was found to be reduced because the region next to the wall dried out prematurely allowing the wall interface temperature to rise well above the saturation temperature. The region above the dried out portion of the wick continued to work with the large pores between the clusters being primarily occupied with vapor and the small pores between the particles being occupied with the liquid. In this work, we report our efforts to reduce the size of the wall-wick interface dry-out region by sintering a thin layer of uniform size particles on the wall as originally suggested in a thesis by Seminic (2007). The boiling curve for this “double layer” wick diverges from a standard “single layer” biporous wick at the point of nucleation by reducing the wall temperature, and concurrently the overall temperature drop across the wick needed to drive a given heat flux. The temperature drop across the wick is reduced because the thin layer of particles between the biporous wick and the wall reduces the wall-wick interface resistance and also provides additional capillary channels underneath the biporous wick. Experimental data supports this hypothesis by showing a clear divergence between measured wall temperatures for the double layer wick from its single layer counterpart. The presumed point of nucleation in both wicks is similar, with the heat flux increasing much more rapidly than the liquid superheat and it is clear that this slope is much steeper for the double layer wick. This finding has great potential to expand the performance capabilities of heat pipes and vapor chambers because the new double layered wick can transfer more heat with less superheat thereby increasing the effective thermal conductivity of the wick and decreasing the wall-wick interface temperature for a given heat flux.Copyright

Utilization of Advanced Working Fluids With Biporous Evaporators

A novel fluid for use as a working fluid in a heat pipe has been tested at UCLA. The fluid was discovered originally in use with a device consisting of a metal tube charged with the patented inorganic aqueous solution (IAS), which is evaporated when the tube is evacuated before use. According to the patent, this evaporation leaves a thin film that allows the tube to carry high heat flux loads with low temperature drop across the tube in a solid state mode. However, various experiments with these tubes have produced inconsistent results, and there are some questions as to whether the fluid is completely evaporated. The research on which this work is based is focused on testing whether the charging fluid will operate as the working fluid in a heat pipe, in order to determine the nature of the IAS fluid. A heat pipe apparatus was charged with a biporous wick in order to investigate if the fluid plays a role in heat transfer. There are extensive data for this experiment using water as the working fluid, which will be used to compare the two sets of results. Testing has shown a reduction of the superheat required to drive heat fluxes through a wick compared to water by approximately 40%. Some experiments have shown that the operating (temperature) range of the IAS is much larger than a standard heat pipe. It is theorized that the increase in performance of the IAS is due to an increased thermal conductivity of the wick and increased capillarity. If this fluid is proven to be effective, it would lead to more effective and tunable heat transfer devices.

Using an Inorganic Aqueous Solution (IAS) in Copper and Aluminum Phase Change Heat Transfer Devices

A novel Inorganic Aqueous Solution (IAS) is shown to have a better thermal performance than water when used as the working fluid in copper or aluminum made heat transfer devices. The effect of each chemical in the IAS and how it benefits heat transfer performance for different materials is explained. It was found that the IAS fluid reacts with copper and coats the surface with a layer of hydrophilic products during the initial boiling process. The surface roughness and wettability were increased which led to an enhanced heat transfer performance. The IAS passivates aluminum surfaces and makes water compatible for use with aluminum heat transfer devices. In addition, IAS has potential to improve the heat transfer performance by 50% lower the superheat when used with non-reactive material heat transfer devices.Copyright

Utilization of Pore-Size Distributions to Predict Thermophysical Properties and Performance of Biporous Wick Evaporators

A sintered copper porous medium is an extremely effective structure used to enhance the evaporative heat transfer properties of a heat pipe. It provides both capillary pressure to passively draw liquid in and increased surface area to more effectively heat the liquid. A biporous wick is particularly effective for this application as there are two distinct size distributions of pores; small pores to provide ample capillary pressure to drive flow through the wick and large pores to provide high permeability for escaping vapor. The modeling described in this work is based on the work of Kovalev who used a pore size distribution in order to determine the most probable liquid saturation at a given position. The model distinguishes phases by choosing a “cutoff” pore size, where larger pores were assumed to be filled with vapor and smaller pores were assumed to be filled with liquid. For a given thickness and thermophysical properties of the liquid, this 1-D model predicts the temperature difference across the wick for a given input power. The modeling proposed in this work yielded results that compare very well with experimental data collected on biporous evaporators by Semenic.

Use of an Inorganic Aqueous Solution to Prevent Non-Condensable Gas Formation in Aluminum Heat Pipes

Heat pipes are used in many applications as an effective means for transferring heat from a source to a sink. The basic heat pipe typically consists of a solid metal casing within which a working fluid is sealed inside at a given pressure. The latent heat transfer via the heat pipe’s working fluid allows it to carry a larger amount of heat energy than would normally be possible with an identically dimensioned solid metal rod. Water is often used as a working fluid due to its high heat of vaporization and suitable operating range for electronics cooling. For many applications, especially space, aluminum is desired as a casing material for its high thermal conductivity, low weight, and low cost. However, water is incompatible for use with aluminum heat pipes because it forms a non-condensable gas (NCG), hydrogen, when they contact. In this work, an inorganic aqueous solution (IAS), which has thermophysical properties similar to water, has been used as the working fluid with an aluminum alloy 5052-H2 casing. The prepared thermosiphon underwent long-term lifetime testing and the results indicate no tube failure or significant NCG formation for the duration of the 9 week study. Furthermore, the data indicate that the IAS fluid not only inhibited NCG production but also led to a reduction in heat pipe thermal resistance over time. It is believed that the chemicals in IAS react with the aluminum surface to create a compact oxide layer and electrochemical reaction which prevents hydrogen generation. A secondary, hydrophilic surface coating is also generated by the fluid on top of the first oxide (passivation) layer. This hydrophilic layer is believed to be responsible for the heat transfer enhancement which was observed during testing and the reduction in ΔT (defined as Tevap−Tcond) over time. Aluminum heat pipes used currently in practice utilize ammonia, or other non-water based working fluids, which have inferior latent heats of vaporization compared to water or an aqueous-based fluid such as IAS. The use of aluminum heat pipe casings in combination with a water-based fluid such as IAS has the potential to provide a significant increase in heat transport capability per device unit mass over traditional ammonia charged aluminum heat pipes.Copyright

A Novel Inorganic Aqueous Solution and its Effect on Liquid Spreading and Freeze/Thaw Processes

Frozen startup of phase change heat transfer devices is a complex problem that can have a large impact on heat transfer systems. A patented novel working fluid developed at UCLA comprised of an inorganic aqueous solution (IAS) was investigated for potential effects on the freeze/thaw capabilities in phase change heat transfer devices by examining the melting process of droplets. Preliminary visual tests were conducted to gain insight into any physical processes that surface augmentation created by this fluid may have on the freezing and melting process. These tests demonstrated significant differences in liquid spreading, the melting process, and the melting rate of droplets on surfaces pre-treated with IAS. Contact angle measurements exhibited enhanced wetting properties. SEM images of frozen droplets showed that liquid freezes in the small capillary wick formed by the initial evaporation of IAS. Video of melting droplets showed a significant increase in melting rate when the surface was first treated with IAS due to superior liquid spreading.© 2013 ASME

Comparison of Vacuum Chamber Tested Biporous Wicks With Thermal Ground Plane Testing

Investigation of bi-porous wicks has yielded an effective method for increasing surface heat transfer when the heat flux is high. It was further found that addition of a mono-porous layer on the heated surface significantly reduced the heated wall surface temperature. These bi-layer wicks were designed for use in 3″ ×5″ heat spreading devices called Thermal Ground Planes (TGP) in order to transfer heat from a 1 cm2 source. In this work we will investigate the performance of a biporous wick with a monoporous layer in various test set-ups to show the versatility of this heat pipe-substrate. Tests were performed at UCLA and at Advanced Cooling Technologies (ACT) to investigate the wick. Experiments at UCLA were conducted in a vacuum chamber setup to isolate the performance of the wick whereas at ACT the wick lined the evaporator side of a TGP. In order to more closely simulate the operating conditions in a TGP and characterize the vapor spacing parameter, some tests at UCLA were performed with a restrictor plate above the wick similar to the space above the wick in the TGP. The data collected using both these experiments showed similar trends of performance as a function of the spacing above the wick. The motivation of this paper is then to validate that the two testing methods provide similar results while independently addressing different parameters.Copyright

Characterization of Vapor Escape Restriction in Biporous Wicks With Monolayers for Thermal Ground Plane Optimization

Developing better heat pipes requires advancement of technology in all aspects of construction. In this paper I am investigating the effect of vapor pathways on the performance of biporous wicks in heat pipes. Biporous evaporator wicks, generated by sintering copper particles into semi-uniform clusters, were demonstrated to achieve high flux, heat transfer performance for use in heat pipes by Semenic (2007). The effective thermal conductivity of thick biporous wicks at high heat fluxes was found to be reduced because the region next to the wall dried out prematurely, allowing the wall interface temperature to rise well above the saturation temperature. One possible way to reduce the size of the wall-wick interface dry-out region is to sinter a thin layer of uniform size particles on the wall as suggested by Seminic. The boiling curve for this “double layer” wick diverges from a standard “single layer” biporous wick at the point of nucleation by reducing the wall temperature, and concurrently the overall temperature drop across the wick needed to drive a given heat flux. The temperature drop across the wick is reduced because the thin layer of particles between the biporous wick and the wall reduces the wall-wick interface resistance and also provides additional capillary channels underneath the biporous wick. Experimental data supports this hypothesis by showing a clear divergence between measured wall temperatures for the double layer wick from its single layer counterpart with an indication that smaller cluster sizes in the biporous wicks perform better at lowering the superheat required to obtain high fluxes. In this work, we are looking to compare the performance of these wicks to similarly sized blocks of copper in order to investigate the performance increase offered by the wicks. In order to investigate this phenomenon we ran experiments in a similar manner to previous experiments done by Reilly (2009), but a plate was inserted into the chamber above the wick to restrict the vapor flow. To determine the behavior in the copper we ran several simulations in COMSOL (a finite element software used for doing conduction analysis) of copper disks at different representative thicknesses. We ran experiments with the plate at several heights above the wick, going so far as to place the plate flush with the upper surface of the wick to force vapor back through the wick laterally. By comparing the results between these two sets of experiments we were able to deduce that even in the case where there was no open space above the wick for vapor to escape, we were still able to double the performance with respect to a system of solid copper.© 2009 ASME

A novel, autonomous thermal connector

A novel replacement for traditional wedgelocks used to mount PCB boards to cold plates is presented. This project began as part of a DARPA design competition to develop a field reversible thermal connector that could be repeatedly assembled and disassembled without the use of tools while providing constant thermal resistance. The team from UCLA was tasked with designing a new device to meet these constraints. The design meets the DARPA goals and significantly reduces the thermal resistance between the electrical board and the heat sink. The device consists of opposing aluminum wedges, driven by thermally actuated Nitinol springs, which slide against one another to provide the requisite locking force to hold a board in place and decrease contact resistance between the interfaces. These smart material springs push the wedges towards the outside of the device, elevating the upper surface and locking the board in place on the cold plate. The design increases the contact area between the components and decreases the thermal resistance relative to current devices. Experimental results have shown that the UCLA team has addressed the chief design problems posed by the REVCON program. Nitinol has been shown to be an effective material for use as a thermally actuated spring capable of repeatedly engaging and disengaging without the use of tools. The wedge locking force increases with rising temperature, enhancing its thermal performance. The UCLA design has been shown to out-perform similar sized current state of the art wedgelock designs in terms of thermal resistance. Reductions in thermal resistance of 30%-45% have been demonstrated and shown to be repeatable. Our prototype design increases the interfacial contact area which has a dramatic impact on performance. This means that higher power density electronics can be utilized or that more real estate will be made available on current computer boards in order to maintain current performance. Further, the use of thermally actuated leaf springs removes the need for mechanical force for installation allowing for less installation time.

Statistical Based Estimation of Biporous Evaporator Dryout

Biporous wicks are an effective means for facilitating evaporation in heat pipes used for electronics cooling. They facilitate boiling within the wick by having two distinct size distributions of pores; the smaller pores provide high capillary pressure to pump liquid to the surface while the larger pores maintain high vapor permeability. The wicks investigated in this study were sintered copper biporous material. The authors previously presented a validated statistical model, based on work by Kovalev, which could predict the performance of biporous wicks tested at UCLA with reasonable accuracy [1]. Using this model, the author was able to gain new insight into the effect that the numerical estimate of liquid saturation of the wick has on dry out. The pore size distribution allows the determination of the capillary pressure available inside the wick and the Kovalev model provides the required pressure drop to supply liquid water to the heater surface. This led to a method of predicting dry out by comparing the capillary pressure in the wick to the required pressure drop from the model to estimate when the wick was dried out.When the required pressure drop determined by code exceeds the peak effective capillary pressure provided by the wick, the large pores of the wick are considered to be dry. These values are correlated to the input heat flux to determine what at what input power the wick begins to dry out. While the wick will not fail in this mode, the overall heat transfer coefficient will have peaked. In this work, this method of determining dry out will be validated against wicks tested at UCLA by comparing the input powers at which this dry out phenomenon occurs. Accurate predictions of dry out and the role of the pore size distribution are critical in developing methods to delay dry out of biporous wicks. By comparing the relative dry out points of various wick geometries to each other, augmented wick geometries can be suggested for future work. This modeling tool can lay the foundation for future tailoring of biporous evaporator wicks to specific tasks.© 2013 ASME