Experimental and Theoretical Study on Heat Transfer Enhancement for a Loop Heat Pipe with Bidisperse Wicks

Loop heat pipes（LHPs）are passive heat transfer devices that are high heat removal with low thermal resistance in long transporting distance. At high heat fluxes, the vapor blanket developing in the conventional monoporous wick in LHP reduces heat transfer performance. In the present study, bidisperse wicks are developed and utilized to improve the above-mentioned defect. Clusters are formed by mixing nickel powder with the three-component binder consisted of polypropylene, paraffin wax and stearic acid. The Bidiserpse wicks are made by sintering the packed clusters. The impact of pore-size distributions in bidisperse wicks on heat transfer performance is investigated by experimental as well as mathematical analysis.
An improved LHP steady-state model was developed. The effect of pore size distribution on evaporative heat transfer was taken into account. The results showed that the comparison between the predicted results and experimental data were within 15% of the mean absolute percentage error (MAPE). The feature of this model is that it quantitatively evaluates the impact of vapor blanket on performance by the concept of average boundary pore radius in monoporous as well as in bidisperse wick. Moreover, the results of the model calculation also showed that bidisperse wicks were affected less by vapor blanket than the monoporous wick and would improve the heat transfer capacity of a LHP.
In experimental results, the pore size distributions of bidisperse wicks can be adjusted by different cluster size. Experimental data indicates that the cluster size has significant effect on performance. Bidisperse wick with smaller cluster size possesses small pore ratio and better heat transfer performance. The cluster size and pore ratio in the best bidisperse wick are 20-62 μm and 13:1, respectively. It is worthy to mention that the pore size ratio in bidisperse wick is important to heat transfer performance when the permeability and porosity are similar with other bidisperse wicks. The evaporator heat transfer coefficient of bidisperse wick reaches the maximum value of 23.3kW/m2K, which is approximately 230% of the monoporous wick at heat load of 400W. On the other hand, the maximum allowable heat load and minimum thermal resistance of bidisperse wick are 575W and 0.127oC/W, respectively. Compared with monoporous wick, bidisperse wick enhances the maximum heat load for 140% and reduces thermal resistance for 80%.
Besides, in order to improve the defect of the bidisperse wick caused by vapor heat leakage at the higher heat loads, the present study builds bi-layer wick structure using the bidisperse wick（cluster size：20-62 μm） as the primary layer and the monoporous wick as secondary layer, respectively. The heat transfer test results show that the maximum evaporator heat transfer coefficient is 35.6kW/m2K, which is approximately 350% of the monoporous wick. The maximum allowable heat load and minimum thermal resistance are 190% (750W) and 50% (0.081oC/W) compared with monoporosu wick, respectively.
To summarize this study, bidisperse wick and bi-layer wick structures effectively enhance the heat transfer performance of loop heat pipe. It is highly potential for the high-power thermal management applications.