Flow Condensation In The Passive Cooling System And Its Effect On The Heat Transfer Characteristics Of The System

When a large amount of heat was accumulated on the power electronic equipment, its working temperature would increase and the performance characteristics would get worse. The stability and reliability of the system became poor. Phase change cooling could enhance heat transfer using the latent heat of vaporization of liquid under low temperature difference. So it can meet the requirements of high heat flux cooling. Two phase flow could be observed in the steam pipe of passive cooling system and this may have influence on the performance of the system.With the help of electrical capacitance tomography (ECT) and high-speed camera, visual investigation showed that a lot of droplet could be observed everywhere on the inner wall of the heat transport pipeline and most of the cross-section area of the tube was free for the vapor flow. The droplet would depart from the inner wall and flow down to the liquid pool at the bottom of the tube due to gravity. This flow pattern was a kind of steady flow pattern, known as the droplet flow. Dropwise condensation was a periodic or cyclic process and each sub process could affect one another. Compared to the Mandhane flow pattern map, the flow regime in the pipeline was mainly stratified flow regimes. Only a few experimental points moved towards the slug and wavy flow zone.There were a lot of formulas for calculating void fraction. The slip ratio model gave the best prediction for the experimental data and how to choose the slip ratio was important for calculation. An improper selection would lead to large deviation. In this paper, we developed a new correlation with sample form, less empirical parameters and wide application range for predicting the void fraction. The vapor quality was further measured accurately by condensing and weighing.The flow resistance during flow condensation under low mass flow rate was measured. According to the two phase distribution and the actual flow characteristics, the theoretical analysis of pressure drop was conducted and we considered the droplet as the wall roughness. We also compared the new model with existing ones to prove its applicability and extend the range of application. The temperatures of wall and vapor along the heat transport pipeline were measured. Combined with visual investigation, the corresponding flow and heat transfer model was developed to analyse the heat transfer characteristics of vapor during flow condensation and predict the heat transfer coefficient. We obtained new correlations for calculating pressure drop and heat transfer coefficient. The root mean square deviations for predicting pressure drop and heat transfer coefficient were 19.9% and 22.04%, respectively. All the new correlations predicted the experimental data within the range of±30%.We chosen the surface temperature of heat source and the thermal resistance of cooling system as the characteristic parameters of the heat transfer characteristics of the system under steady conditions. Meanwhile, the response time represented the operating performance of the system under unsteady conditions. When the heat transfer coefficient of the steam pipeline increased, the temperature drop of the vapor increased and thus the condensate temperature became lower, which was helpful to decrease the surface temperature of heat source. But the thermal resistance was relatively large. The thermal capacity of the vapor increased due to the accumulation of the water steam in the evaporator and pipeline. The system needed more time to reach the steady-state again and thus the response time of the system was long.In this paper, we selected the passive phase-change cooling system as the basic model and focused on the study of flow condensation in the adiabatic section or steam pipe. We also revealed the coupled law of flow and heat transfer during in-tube condensation under low mass flow and described the effects of the geometry size and placement on the flow condensation. At last, we obtained the influence mechanism of the effect of flow condensation on the operating performance and the heat transfer characteristics of phase-change cooling system.The research findings in this paper will enrich the study of flow condensation and provide guidelines for the optimal design of pipelines in the cooling system.