Experimental investigation of pulsating heat pipes with different evaporator roughness values

Abstract

Three transparent experimental setups of flat-plate pulsating heat pipes were investigated, each with a different channel surface roughness in the evaporator section. Roughness was varied using milling and sand blasting techniques. Experiments were conducted in different orientations with the refrigerant R1233zd(E) with filling ratios of 30%, 50% and 70%. Heat flux values between 5 and 45 W and condenser temperatures of 0 °C, 20 °C and 40 °C were applied. Low filling ratios, high condenser temperatures and high heat flux values were found to achieve the lowest thermal resistances. Increased surface roughness in the evaporator section also reduced the thermal resistance. The effect was especially pronounced for low heat flux values. The surface roughness in the evaporator section was analyzed with a confocal and a novel evaluation method with particular regard to the influence of the surface structure on the bubble nucleation behavior was introduced. The evaluation method yielded estimated bubble radius distribution profiles of the cavities found in the surface structure. The data was then used to calculate a mean bubble nucleation radius for the surface structure of each test setup. Combined with the pool boiling nucleation equation this resulted in a theoretical relative bubble nucleation threshold calculated for each measurement point. It was found that the theoretical bubble nucleation threshold could be directly correlated with the thermal resistance and overall performance of the pulsating heat pipe. In the experiments, bubble nucleation rates were determined by evaluating high-speed camera recordings using a bubble tracking algorithm. For a 70% filling ratio the bubble nucleation rate was found to increase up to a maximum with increasing heat flux. When approaching high heat flux values the bubble nucleation rate dropped again. The quantification of the connection between bubble nucleation threshold and thermal performance is a novelty in this work, as well as the quantification of the bubble nucleation rate in an experimental setup. The work offers meaningful insights into the thermo-hydraulic behavior of a PHP.