Experimental Investigation And Optimization Of Thermal Performance Of Silicon-based Micro Heat Pipes

One of the most prevalent passive thermal solutions for semiconductor devices is micro heat pipe(MHP).In this dissertation,various silicon-based MHPs were designed and fabricated by micro electro mechanical systems(MEMS)methods to investigate thermal performance of MHP.Fundamental heat transport of MHP is relied on two mechanisms:phase-change heat transfer of the working fluid and heat conduction through the solid wall.Hence,working fluid and capillary structure are two prominent parts of MHP.The micro grooves in MHP plays role of capillary structure and deals with producing the capillary pumping pressure to return condensed fluid to evaporator.So,the shape(cross-section)and dimensions of micro grooves make influence on MHP’s efficiency.To investigate effect of micro grooves’ shape on thermal performance of MHP,three silicon MHPs with various cross-sections of(a)trapezoid with rectangular arteries,(b)rectangular,and(c)rectangular with arteries,were fabricated.Fabrication of arteries by bonding two or three wafers had been reported.However,in our work,by using MEMS fabrication methods three MHPs with same hydraulic diameter were fabricated on only one silicon wafer.The arteries were increased void fraction of micro grooves and number of sharp corners of micro groove to improve capillary pressure.DI water with high merit number was selected as working fluid.To investigate effect of filling ratio on performance,each MHP was charged in three levels of 20%,40%and 60%.The final results concluded that the trapezoid with rectangular arteries and filling ratio of 40%were achieved the best effective thermal conductivity.Design of MHP is a complex function of various structural and operational parameters that results in the non-linear problem which it is complicated to solve.Type of cross-section,filling ratio and input power were three independent variables(input variables)which had prominent effects on effective thermal conductivity of MHP as target variable(output variable).In addition,each input variable had different levels that were caused difficulties to find the optimal condition.Hence,Box-Behnken design(BBD)model from response surface model(RSM)family was used to estimate the fittest value for effective thermal conductivity of MHPs.According to the selected input variables and their levels,the BBD model was defined the minimum sets of experiments.Based on the achieved thermal conductivities which were measured from defined experimental sets,the BBD method was developed an optimal mathematical model with the reasonable fitness to evaluate the effects of input variables and their interactions.This model predicted the amount of effective thermal conductivity close to achieved results from experiments.The optimal condition was defined through BBD model was trapezoid with rectangular arteries’ structure and filling ratio of 40%which was compatible with experimental results.The groove and fin dimensions also make significant effects on the performance of MHP.On the other hand,the optimal design of MHP always engages with trades-off between competing parameters of pressure drop and heat transfer coefficient.To investigate the mentioned parameters,six different silicon-based MHPs with rectangular cross-section were fabricated in two groups:First group was micro grooves with different fin width and second group was micro grooves with various grooves width.Based on the measured temperature of DI water in different sections of evaporator,adiabatic and condenser,the physical properties of DI water was achieved and by using the theoretical formulas the pressure fluctuations of liquid and vapor phases,as well as,heat transfer coefficient of evaporator and condenser in various input powers were analyzed.Furthermore,the thermal network modeling was done for all MHPs and effective thermal conductivity was calculated.The results were demonstrated that in first group,the MHP with smaller fin width and in second group,the MHP with bigger groove width had better performance.Comparison of two groups was showed in first group,with increasing the number of grooves total performance of MHP was enhanced,but in second group this trend was in inverse.In our case,the optimal number of micro grooves was about 60-70 and resulted in higher effective thermal conductivity of MHP.Although micro channels have inherent characteristics of high surface area to volume ratio,smaller volume and mass,and high convective heat transfer coefficient,the flow boiling in micro channels deals with flow instability and low values of critical heat flux(CHF)challenges.Because the bubbles generated from boiling incipience of the liquid coolant on the superheated surface are confined between the surrounding walls with decreased channel dimensions.To overcome this challenge,micro pillars were used to increase the surface area to volume ratio and nucleation sites.So,micro pillars with different shapes of circle,square and diamond,from two materials of silicon and copper,in two patterns of straight and zigzag were fabricated inside the micro axial grooves of MHPs.The capillary rise test was used to evaluate the wettability of micro grooves.The final results showed the wettability and thermal performance of MHPs with micro pillars were superior to MHPs without micro pillars.Additionally,the wettability and thermal performance of MHPs with copper micro pillar,circle shape and zigzag pattern were preferable.