Critical heat flux in a 0.38 mm microchannel and actions for suppression of flow boiling instabilities

Abstract

This paper presents experimental results for critical heat flux in a 0.38mm internal diameter tube during saturated flow boiling. Experiments were performed for refrigerant R134a flowing inside a horizontal stainless steel circular channel of 70mm heated length, mass velocities ranging from 200 to 1400kg/m2s, saturation temperature of 31°C and critical heat fluxes up to 215kW/m2. A parametric study of the effect of mass velocity revealed the same trends observed in previous studies with 1.1 and 2.2mm internal diameter tubes. In contrast, prediction methods that performed well in previous works for internal diameters higher than 1.00mm failed to predict the data for the 0.38mm tube. An investigation into the reasons for the failure of these methods revealed thermo-hydraulic instabilities are more pronounced for the 0.38mm tube and actions to cancel these effects are required. Tests revealed that a saturated inlet vapor quality near 5% combined with a high inlet pressure drop could increase the critical heat flux up to 50% in comparison to the results without any control actions. Moreover, conventional CHF predictive methods from the literature provided a reasonable prediction of the results for the 0.38mm tube when instability effects were minimized.