Study On Dynamic Thermal Characteristics Of Pumped Two-Phase Flow Loop System

With the continuous development of science and technology,the performance and integration of various types of electronic equipment have been continuously improved,which has promoted the thermal management of electronic equipment to develop in the direction of high power consumption and high heat flux density.For equipment that is sensitive to changes in thermal stress,it is also necessary to ensure that the temperature uniformity and temperature fluctuation range of the heat source meet the requirements,so as to ensure the normal and stable operation of the electronic equipment and even the entire system.Therefore,the pumped two-phase flow loop system under active temperature control based on the two-phase flow’s efficient heat dissipation and phase change constant temperature characteristics has gradually become a research hotspot.The microchannel has the characteristics of simple structure,small volume,and large heat transfer specific surface area,which is especially suitable for high heat dissipation and precise temperature control of high heat flux heat sources in limited spaces.In the actual working process,microchannel heat transfer often faces problems such as working condition conversion,thermal load changes and external thermal disturbances,which forces us to grasp the dynamic characteristics of two-phase flow heat transfer in microchannel.However,the current research on this type of system is mostly focused on system performance testing,structural optimization and control strategy improvement under steady-state conditions,and lack of exploration of the system’s thermal dynamic characteristics.Therefore,this article built a pumped microchannel two-phase flow loop system to study it’s thermal dynamic characteristics,and analyzes the dynamic changes of the system from multiple angles such as temperature,pressure and flow pattern.Thermal dynamic characteristics mainly include startup characteristics and thermal response characteristics,and compare the differences between different operating strategies.In summary,the main research contents and conclusions of this article include the following parts:(1).The performance test platform of the pumped two-phase flow loop system was designed and built,and the visual experimental study of the microchannel flow boiling process was carried out.According to the law of temperature change,the system’s startup mode is divided into three types: gradual startup,once-rise overshoot startup and twice-rise overshoot startup,and the causes of various startup modes are explained by the mutual verification of temperature and flow pattern.The wall temperature of the gradual startup increases monotonically,mainly because the flow pattern and heat transfer mechanism in the startup mode change slowly.In this mode,the heat transfer flow pattern changes from single-phase flow to bubble flow to slug flow,and the corresponding heat transfer mechanism also changes from single-phase convection to nuclear boiling.The flow pattern and heat transfer mechanism of the overshoot startup are changed drastically,and the wall temperature will show a sudden drop process.What’s more,there will be a second rise process after the temperature drop for the twice-rise overshoot startup;On the one hand,the heat transfer flow pattern initiated by the once-rise overshoot startup quickly transitions from bubble flow to slug flow or elongated bubble flow during the overshoot,and the steady-state heat transfer mechanism is mainly nuclear boiling;On the other hand,the heat transfer flow pattern initiated by the twice-rise overshoot startup during the overshoot are consistent with the one-rise overshoot startup.However,with the second increase in wall temperature,the heat transfer flow pattern gradually transitions to agitated flow or annular flow,and the heat transfer mechanism at steady state includes nuclear boiling and convective evaporation.According to the startup modes under different working conditions,the dimensionless Jakob number and We number are used to draw the startup state diagram quantitatively.For different starting modes,corresponding evaluation indicators are adopted and the influencing factors of each indicator are analyzed.(2).The dynamic response characteristics of the pumped two-phase flow loop system are explored from the perspectives of flow pattern,temperature,pressure,etc.,and the effect of thermal load on thermal dynamic characteristics is mainly analyzed.Within the range of full load change,the heat transfer flow pattern of the microchannel presents bubble-like flow,slug flow,elongated bubble flow,agitated flow,annular flow and then reverse annular flow with the increasing heat load.The heat transfer deterioration starts from the exit of microchannel and gradually evolves to the entrance of microchannel,and the reverse annular flow is the sign of heat transfer deterioration.Accordingly,with the increase of heat load or the decrease of circulation flow,the reverse annular flow also extends from the exit to the entrance.As for pressure aspect,the pressure at each point of the system gradually increases with the increase of heat load,and the pressure at the outlet of the evaporator is basically unchanged after reaching the high load area.The pressure change at the inlet of the evaporator within the full load range can be divided into four phases: single-phase stable zone,two-phase unstable zone,two-phase stable zone,and critical heat exchange unstable zone.What’s more,the pressure drop of the evaporator is directly proportional to the heating load,increasing the system’s circulating flow,the slope of the pressure drop curve remains the same,but the pressure drop value under the same heat load increases.From the perspective of system heat transfer efficiency,the average heat transfer coefficient of the evaporator showed a rapid rise,a uniform rise,and a rapid decline in three stages with the increase of heat load.The working fluid exchange between the liquid storage tank and the loop during the dynamic change of the system can be judged by the dynamic change of the temperature measurement point on the connecting pipe.When the circulation flow rate of the system decreases or the heating load increases,the temperature of the measurement point decreases,and the temperature of the measurement point near the loop is the lowest,indicating that the working fluid is flowing to the liquid storage tank,otherwise it flows out of the liquid storage tank.(3).According to the characteristics of each instrument,a set of automatic control system that integrates data transmission,actuator control and temperature regulation is designed,and based on the control system,the performance characteristics of the system under different operating strategies are explored.Flow control strategies can be divided into active flow control and passive flow control.The average heat transfer coefficient of active flow rate control at constant inlet temperature is lower than that of passive flow rate control,but the average wall temperature is opposite,and the result at constant inlet subcooling is just the opposite of that at constant inlet temperature.Comparing the two flow control methods,the active flow rate control has poor temperature uniformity,and the passive flow rate control has a slight increase in temperature uniformity before the heat transfer deteriorates.Based on this system,there is an optimal flow rate interval in terms of flow rate control.On the premise of constant subcooling,the circulation flow rate on the right side of the optimal section is large,meanwhile the average heat transfer coefficient and average wall temperature of the evaporator have increased,but the energy consumption of the system has increased significantly.However,the circulation flow rate on the left side of the section is too low,and the dryness of the evaporator is too large,which leads to the deterioration of heat transfer.Therefore,the average wall temperature rises rapidly,the temperature uniformity is greatly reduced,and the system cannot work normally.In this interval,the average wall temperature of the system is the lowest and the temperature uniformity is the best.What’s more,the average heat transfer coefficient is not much different from the right side of the optimal interval,but the energy consumption of the system is greatly reduced.On the other hand,there is a turning point in the pressure drop at the inlet and outlet of the evaporator under a constant heating load,which is called onset of flow instability(OFI).The heat source temperature can be effectively controlled by adjusting the temperature control unit on the liquid storage tank to change the back pressure of the system to affect the saturation temperature of the evaporator.The research work of this paper provides practical data for the design and operation regulation of pumped two-phase flow loop system,which will provide technical support for the dynamic thermal management of high power consumption,high heat flow density and high temperature control accuracy heat source.