High flow rate electrospray cooling performance of water–ethanol mixtures

Abstract

Electrospray (ES) is an effective approach for cooling high-power density devices, owing to its desirable features such as uniform droplet diameter, suppression of droplet rebound, and low power consumption. Despite its excellent thermal properties, water has not been extensively employed as a working fluid in ES setups due to difficulties in establishing a stable ES mode. In this study, ES cooling performance of high flow rate water–ethanol mixtures is investigated. The stable region of the cone-jet mode is identified by capturing the meniscus profile and detecting the corona discharge. The study investigates the effect of ES variables, including flow rate (20 mlhr −100 mlhr), applied voltage (5.30 KV − 12.15 KV), and working fluid including pure ethanol (xH2O= 0 %), mixture I (xH2O= 35 %), and mixture II (xH2O= 70 %), on the cooling performance within the stability region. The results show that raising the flow rate from 20 mlhr to 100 mlhr augments the onset voltage of the cone-jet mode in each fluid. In the case of pure ethanol and mixture I, the end voltage corresponding to the multi-jet mode increases with the rise in flow rate. However, As a result of the heightened surface tension, no multi-jet mode is observed for mixture II, and only the occurrence of corona discharge defines the end voltage. In the single-phase regime, the cooling behavior only depends on the flow rate, owing to the thick liquid layer formed on the heated surface. However, it has been revealed that when the mixture water volume fraction increases up to 70 %, the critical heat flux rise is approximately82 % (Q = 20 mlhr), 148 % (Q = 60 mlhr), and 167 % (Q = 100 mlhr), at the onset voltage and 98 % (Q = 20 mlhr), 131 % (Q = 60 mlhr), and 147 % (Q = 100 mlhr), at the end voltage due to the improved thermal properties of the fluid. Next, the effect of each ES variable on cooling performance is investigated separately. Based on the results, an increase in the flow rate from 40 mlhr to 80 mlhr leads to an approximately 70 % rise in the critical heat flux for both mixture I and mixture II. Additionally, increasing the mixture water volume fraction from 35 % to 70 % results in an approximately 29 % rise in the critical heat flux, attributed to the larger latent heat of mixture II. In the transition regime, mixture II demonstrates superior performance compared to mixture I by suppressing the rebounding effect and enhancing the cooling performance. This improvement is due to the rise in the image force caused by the increase in water volume fraction. However, this enhancement is more significant at the lower flow rate due to the shorter impact time.