High Heat Flux Removal HOME > RESEARCH > Phase Change Phenomena> High Heat Flux Removal
 
Background

The flow boiling in tubes and channels is the most complex convective phase change process encountered in many applications (e.g., evaporator, boiler, and nuclear/fusion reactor components etc.). Fig. 1 shows the flow boiling regimes for vertical channel in low and high heat flux level. For flow boiling in vertical tubes at the relatively lower heat flux, boiling may be initiated before the bulk liquid reaches the saturation temperature. As the vaporization process continues, and liquid is converted to vapor, the flow regime progressively changes from bubbly flow to slug flow, slug flow to churn flow, and churn flow to annular flow. On the other hand, for flow boiling in vertical tubes at very high heat flux, vapor formation takes place in the presence of subcooled liquid. As the vaporization process continues, the flow regime changes from churn flow, DNB (departure from nucleate boiling) and dryout to inverted annular flow. At the downstream of the heating tube, liquid column in the center of tube is separated by velocity difference between liquid column and vapor film. (inverted slug flow or inverted churn flow) Finally, liquid droplets separate into the smaller droplets. (inverted bubbly flow)

Fig.1. Regions of heat transfer in convective boiling (김무환 외 2명, Two-Phase Flow Heat Transfer, 1993).
 
Objectives
Our goal is to verify the performance of a newly designed coolant channel with 30 MW/m2 of high heat flux cooling system. Parametric studies are conducted to estimate heat transfer performance from various channel geometries.
- Enhancement of the heat transfer efficiency and critical heat flux (CHF)
- Optimization of channel structure for heat transfer performance and pressure drop
- Investigation of heat transfer enhancement mechanism from designed channel
Method

In this study, the cooling limit of steady state is assumed to be 30 MW/m2 to secure the thermal margin of the divertor cooling system. The mechanism of the critical heat flux is divided into the far field and the near field depending on the hydrodynamic region of interest. In the previous research, it has been optimized by concentrating on one mechanism, but it is considered that two mechanisms should be considered to efficiently remove the heat flux assumed by the fusion reactor and secure the stability of the system.

• Far field: When the phase change occurs, the bubbles stay on the heating surface for a certain period of time and then disappear after growing, so that the bubbles block the flow of the surrounding liquid to the heating surface. The critical heat flux can be improved by controlling the release period of the bubbles at a high speed, or by controlling the escape of the bubbles and the flow path of the liquid. This study aims at maximizing cooling water inflow and bubble departure flow through secondary flow control using Hypervapotron fin shape.
• Near field: The thin film under the bubbles formed on the heating surface irreversibly dries to generate the critical heat flux. By applying the micro-structure, it induces the inflow of liquid by the capillary phenomenon and suppresses the growth of the dry region causing the critical heat flux phenomenon. The strategy of this study is to apply the micro structure to the surface of the fin to induce the capillary phenomenon and to maximize the heat transfer area, and the capillary force between the microstructures causes additional cooling water inflow between the structures.

Fig. 2. Heat transfer performance enhancement strategy through milli- and micro-structure.
Experiment

Flow boiling loop contains test channel, heat exchanger, accumulator, main pump, booster pump and SCR power supplier. The test channel is heated by direct heating method. Heat flux is controlled by SCR which regulates supply of electric power. Water flow rate is controlled by main pump and regulated accurately from bypass line. Measuring equipment includes thermocouples, pressure transducers, pressure gauge, differential pressure gauge and turbine flowmeter. Data acquisition system (DAS) acquires signal from these devices and converts to temperature, pressure and flow rate. Heat transfer performance at very high heat flux is investigated through this experimental loop (Fig. 4). Maximum condition of inlet temperature and pressure are 150 °C and 24 bar, respectively.

Fig. 3. Experimental facility.