Research On The Heat Transfer And Thermal Stress Problem Of Water-cooled W/Cu Monoblock For Divertor

The controlled fusion energy is a long-cherished goal for human. Tokamak is the most important in all the fusion devices. The divertor is the key component used to exhaust part of the plasma thermal power as well as to minimize the impurity content in the plasma for tokamak. The water-cooled W/Cu monoblock for divertor is subjected to high heat flux, up to several MW/m2 in a steady state and hundreds of MW/m2 in a transient state, on its one side facing the plasma. Under such high heating condition, the water-cooled W/Cu monoblock will be destroyed by too high temperature under working condition, heat transfer deterioration and oversize thermal stress in general. Therefore, the accurate prediction method and design criteria of heat exchange efficiency, critical heat flux(CHF) and thermal stress of water-cooled W/Cu monoblock are crucial for its engineering design. Based on the brief theory mentioned above, the research contents and conclusions in this paper are as follows:Firstly, relaxation coefficients were corrected by the formula Hertz-Knudsen in literature in order to improve the calculation accuracy of vapor-liquid conversion efficiency under high heating flux condition. Moreover, local large differences of liquid physical properties due to the extreme nonuniform heating flux of cooling wall along the circumference direction were revised by formula IAPWS-IF97. Based on Euler homogeneous model and above corrected method, subcooled flow boiling heat transfer characteristics of water-cooled W/Cu monoblock under different flow and geometric parameters were investigated. The results had showed that: there were different heat transfer modes of the cooling wall along the circumferential direction, namely, the subcooled boiling in the upper part of the cooling wall and the single-phase convection in the lower part. It was the main characteristic which was different from other heat transfer equipment. The modes of heat transfer in different region of cooling channel were changed with variable flow parameters. The heat transfer of cooling wall became worse with decreasing velocity. The lower coolant supercooling led to the worse heat transfer of the top of the cooling wall, however, the heat transfer of the middle and lower part of cooling wall became better meanwhile. With increasing heat flux, the pressure drop ?p along the cooling channel decreased first and then increased. With increasing coolant supercooling and increasing inlet velocity, ?p increased. The temperature of monoblock increased with increasing width of monoblock, and the strength of subcooled boiling heat transfer was enhanced meanwhile. With increasing armour thickness, the temperature of heated wall increased significantly, and the heat flux was more focused on the top of the cooling channel.Secondly, the CHF of water-cooled W/Cu monoblock for divertor were predicted by numerical simulation, and the the effects of flow and geometrical parameters on its CHFs were also been investigated. The results have showed that: there was vapor film in the top of rear part of the coolant channel when CHF occurs, and the vapor in the vapor film was supplemented by the bubbles in the side of top of the coolant channel. The heat convection between liquid and cooling wall was thwarted by the vapor film. The incident critical heat flux(ICHF, the heat flux at the heated surface when CHF occurs) increased with decreasing width of monoblock and armour thickness, with increasing velocity and subcooling. The peaking factor fp was adapted to characterize the use degree of heat removal capability of water-cooled W/Cu monoblock. In this paper, the heat conduction in solid domain of water-cooled W/Cu monoblock was calculated numerically by assuming the local heat transfer coefficients(HTC) of the cooling wall to be functions of local wall temperature, so as to obtain fp. The effects of geometric and flow parameters on the fp of water-cooled W/Cu monoblock were investigated. It was shown that the fp increases with increasing width of monoblock, armour thickness, Reynolds number and Jakob number. Based on the calculated results, an empirical correlation on the peaking factor of a water-cooled W/Cu monoblock for divertor with smooth channel was obtained via regression,Then, the subcooled flow boiling heat transfer characteristics of water-cooled monoblock for divertor were investigated by experiment, and the experimental procedure were numerical simulated meanwhile. The effects of heat flux and inlet velocity on temperature distribution of monoblock and the average HTC of cooling wall were been investigated. The results had showed that: due to the heat transfer enhancement in nucleate boiling regime, the increase of temperature along the flow direction in the rear part of the cooling wall was much slower than that in the anterior part. With increasing heat flux, the average HTC of cooling wall changed slightly in the single-phase regime, but increased significantly in nucleate boiling regime. The average HTC increased with increasing velocity. When velocity was 1m/s, the ICHF obtained by experiment was 130kW/m2.Finally, elastic-perfectly plastic assumption was adopted and the volume fraction distribution of W in functionally gradient materials(FGM) along the radial direction was described approximately by the power law equation. The calculation model for thermal stress calculation of W/Cu FGM for watercooled monoblock prepared by hot isostatic pressing(HIP) was established. The effects of uniform thermal boundary condition on working thermal stress calculation of water-cooled W/Cu monoblock were investigated. The results had showed that: the distribution of thermal stress under uniform thermal boundary conditions shows the same trend as that under nonuniform thermal boundary conditions, but was smaller in value, with the maximum difference of 15.2%. Based on these above, the effects of compositional distribution parameter p, FGM thickness Δ, welding temperature and welding pressure on the residual and working thermal stress of of W/Cu FGM for water-cooled monoblock were investigated by numerical simulation. The results had showed that: optimal thermal stress reduction was obtained for 0.1≤p≤0.2 and Δ≥1mm, and the maximum reduce of von mises residual stress and von mises working stress are up to 53.8% and 43.8% respectively by comparing with non-FGM(pâ†'∞). With increasing welding temperature, the residual thermal stress increased and working thermal stress decreased; with increasing welding pressure, the residual thermal stress decreased and working thermal stress increased. The selection of welding temperature and welding pressure should be synthetic in engineering design.