喷雾冷却系统启动性能的实验研究

李 丽 苗建印

(北京空间飞行器总体设计部,北京 100094)

1 Introduction

To dissipate heat flux from current and future high power directed energy devices for space based applications, advanced thermal management approaches should be adaptable to high heat flux acquisition and high heat rate transport in space enviroment[1].Direct cooling by means of spray has been considered as an effective solution to the problem of cooling high power density directed energy devices since the method is capable of very high heat removal rate (higher than 500 W/cm2for water)although the spray cooling system for the aerospace application is a little complicated[2].

Although spray cooling has long been employed in the quenching of metals[3], it is not applicable to cooling spacecraft instruments until the last decade.

An experimental package of single-phase spray cooling system was fabricated for variable gravity flight tests, which was flow n on N ASA KC-135 Reduced-Gravity Research Aircraft[4-5].Flight test data and terrestrial data were compared with analytical and numerical solutions in order to evaluate the heat transfer in the heater and support structure.Lin, et al.established a closed loop spray cooling test of large cooling area.A multi-nozzle plate with 48 miniature nozzles was designed to generate an array of 4×12 spray cones for the cooling of high power directed energy source components using FC-72 as the working fluid[6].Experimental results showed that C HF for FC-72 in the case of large area spray cooling reached up to 59.5W/cm2.M otorola carried on a series of spray cooling experiments on diode lasers[7-8].The studies have shown that spray cooling with water at reduced system pressure can significantly decrease the surface temperature compared with ambient pressure spray cooling, at a pressure of 0.015 bar,and the surface temperature can be cooled to 54 ℃with 11 ℃liquid at heat flux of 380W/cm2, w hile the surface temperature at 380W/cm2is about 120 ℃with the same liquid temperature at ambient pressure.M uch w ork has been done to determine the critical heat flux (C HF)in spray cooling[9].It is generally accepted that the appropriate nozzle-tosurface distance can be determined, know ing the size of the heater and the spray angle, and the higher CHF values can be achieved by increasing the total spray flow rate and the liquid sub-cooling at the nozzle inlet.

The fluid loop must be successfully started before the spray cooling system begins to serve.And the startup of two-phase fluid loop is a very complex process with phase change, boiling, evaporation and condensation.However, no study has been published on the startup of spray cooling until now.The objective of this paper is to investigate the startup performance of the spray cooling system through a series of closed-loop spray cooling experiments.

2 Experiment Setup

The closed fluid loop is shown schematically in Fig.1. The loop consists of a reservoir, a pump, a preheater, a spray chamber housing a pressure atomizer, heater, and condenser with water as the working fluid.

Fig.1 Schematic diagram of spray cooling system

A gear pump (GB-P23.JVS.A +DB380A Micro Pump Inc.)was used to pump two-phase fluid from the reservoir to nozzle.The condenser is a single-phase loop that maintains the temperature of exterior w all at 9 ℃(temperature of heat sink).

The spray chamber, as shown in Fig.2, consists of a pressure atomized nozzle, heater and pump systems.At pressure drop of 2～3 bar the diameter of spray droplets ranges from 20μm to 30μm with flux rate of 0.4～2GPH.The heater uses multi-layer structure due to its low thermal leakage. Three thermocouples inside the heater support structure were used to determine the heat loss from the heater, as shown in Fig.3, and the heated surface temperature (Tw)is derived from the measured values of TC1, TC2, and TC3.

Transient data were monitored using thermocouples, pressure transducers, and flow meters distributed throughout the experimental apparatus, as shown in Fig.4, where TC means thermocouple and P means pressure transducer.

3 Operating cases

Fig.2 High heat flux heater and vacuum spay chamber

Fig.3 Thermocouple locations inside heater support structure

Fig.4 Location of measured points

Table 1 gives the basic operating modes of startup performance.The environmental temperaturewas about 2 4.5 ℃.The heat sink was started at first in all the cases maintaining the temperature at 9 ℃.According to the gas-liquid distribution in fluid loop before startup, the experiments can be classified into two groups:in case 1 and case 2 little liquid was observed in the spray chamber before startup, however;in case 3 and case 4 volume liquid accumulated in the spray chamber before startup.To study the startup condition of pump in case 1 and case 3 the pump was started before the heater, and in case 2 and case 4 the heater began to serve.

Table 1 Setup of startup modes

4 Results and Discussion

4.1 Little liquid in spray chamber before startup

A t the first experiment, little liquid was observed in spray chamber, experiments of case 1 and case 2 were conducted to invest the startup condition of gear pump.The pump was started before the heater began to w ork in case 1 and the pump was started after the heater w orked for a w hile in case 2.Temperature of the monitored points is illustrated as Fig.5.

Fig.5 Temperature of monitored points vs.time(case 1 and case 2)

Water loop condenser was started firstly .It could be seen that only the temperature of the condenser decreases with the duration of time, which showed that no fluid loop was formed w hen only heat sink started up.After the pump and the heater both were started in sequence, liquid spraying can be observed from the nozzle to the heated surface at first, temperature of the condenser inlet(TC6)increased and temperature of the condenser outlet (TC12)decreased;however, the spraying just maintained for a short w hile and the temperature of outlet and inlet tended to be same;the startup of case 1 failed.In case 2, the heater was started first with a heat load of 50W, w hen temperature of the heated surface (TW)reached up to about 70 ℃, the pump started.It could be seen that the temperatures of the heated surface, inlet and outlet of spray chamber all decreased until the state was balanced, where the surface temperature maintained at about 25 ℃.It showed that the fluid loop generated in the spray cooling system as soon as the pump started and could be sustained.

By comparing case 1 with case 2, as little liquid was observed in the spray chamber before the startup of pump, sufficient liquid the pump must exist in the pump at first, so the fluid loop could start in both cases.However, in case 1, the working fluid could not flow from the spay chamber to the pump because the pressure difference between the outlet and inlet of spray chamber could not overcome the flow resistance, and in case 2, the saturation pressure in spray chamber was dramatically increased by absorbing the heat from the heated surface, so smooth fluid loop was formed and fluid flowed to pump smoothly.

4.2 Volume liquid in spray chamber before startup

Volume liquid may be accumulated in spray chamber before startup, as shown in Fig.6, which is advantage to the startup of pump because the less liquid can be held in the pump.

Fig.6 Volume liquid in spray chamber before startup

Fig.7 show s the startup of case 3 and case 4.There was volume liquid in spray chamber before the startup of both cases.It is obvious in case 3 that pump could not start without being heated in advance, and fluid loop could not operate either.

Fig.7 Temperature of monitored points vs.time(case 3 and case 4)

In case 4, the heater firstly started with the heat load of 50W.About 5min later, temperature of condenser inlet (TC6)increased suddenly,which means that the hot liquid began to flow from the spray chamber into the reservoir and pump.The pump started w hen the surface temperature(Tw)was up to about 42 ℃.After the startup of pump, the temperatures at the locations of TC3,TC2 and TC1 began to be different, it showed that heat flux from the heater was removed by the fluid loop.With the time going on, all temperatures throughout the system tended to be balanced.

By comparing Fig.7 with Fig.5, at the balance state w hen volume water accumulated in spray chamber before startup, the temperature of heated surface was about 38 ℃, which is much higher than that w hen little water was observed in spray chamber.

5 Conclusions

The closed-loop spray cooling experiments have been performed to investigate the startup performance of spray cooling system.The experimental system mainly consists of pump, atomizer on air pressure, heater and condenser.The working fluid water was atomized through a full cone nozzle into a vacuum chamber, and then onto a heater surface.

As gear pump was adapted to drive the twophase fluid loop, sufficient liquid may be needed to infiltrate the pump, which was proved by the experimental results.

In addition to the influence of pump, the spray cooling fluid loop system was also driven by the pressure head and temperature difference, so the low temperature heat sink should firstly start;the heater must start before the pump generates high pressure of high temperature saturated steam in spray chamber.It can be concluded that the startup should be in the sequence of heat sink, heater,and then pump.

The liquid-gas distribution in the spray chamber before startup also affects the starting performance significantly.Volume liquid accumulated in the spray chamber before startup may reduce the efficiency of spray cooling and may cause the increase of heated surface temperature at the balanced state.

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