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

• New critical heat flux experimental results for R134a in 0.38 mm microchannel are presented.
• Critical heat fluxes of up to 215 kW/m2 were obtained.
• Actions to suppress the effects of instabilities in the critical heat flux were tested.
• CHF increased 50% when the flow had a vapor inlet of 5% combined with inlet restriction.
• Critical heat flux decreases decreasing the tube diameter.

This paper presents experimental results for critical heat flux in a 0.38 mm internal diameter tube during saturated flow boiling. Experiments were performed for refrigerant R134a flowing inside a horizontal stainless steel circular channel of 70 mm heated length, mass velocities ranging from 200 to 1400 kg/m2 s, saturation temperature of 31 °C and critical heat fluxes up to 215 kW/m2. A parametric study of the effect of mass velocity revealed the same trends observed in previous studies with 1.1 and 2.2 mm internal diameter tubes. In contrast, prediction methods that performed well in previous works for internal diameters higher than 1.00 mm failed to predict the data for the 0.38 mm tube. An investigation into the reasons for the failure of these methods revealed thermo-hydraulic instabilities are more pronounced for the 0.38 mm 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 0.38 mm tube when instability effects were minimized.