Flow boiling characteristics in a novel type of sinusoidal wavy microchannels with stepped expansion flow passages

•The novel sinusoidal wavy microchannels with stepped expansion flow passages (SWM-SEFPs) are proposed.•The temperature of onset of nucleate boiling for novel microchannels is reduced to 57.36% compared to rectangular microchannels.•The excellent heat transfer performance of the SWM-SEFPs leads to a smaller temperature of the heating film and a higher heat transfer coefficient than the rectangular microchannels.•Stable slug and annular flows can be maintained in the SWM-SEFPs and reduce the wall temperature fluctuations of the heating film as compared to the rectangular microchannels. Novel sinusoidal wavy microchannels with stepped expansion flow passages (SWM-SEFPs) have been developed to enhance the heat dissipation and inhibit the flow boiling instability in the present study. Experiments of flow boiling in rectangular microchannels (RMs), sinusoidal wavy microchannels (SWMs) and SWM-SEFPs were conducted at the mass flux G = 1056 - 2322 kg/(m2s) and the effective heat flux qeff = 0 to 233.68 W/cm2. The working fluid is HFE-7100. The measured flow boiling characteristics in three microchannels are compared and analyzed. The novel SWM-SEFPs can dissipate the heat flux of 202.11 W/cm2 at the wall superheat of 39.12°C, which is 43.4% greater than that dissipated by the RMs at G = 2322 kg/(m2s). The maximum heat transfer coefficient (HTC) of 13.84 kW/(m2K) is achieved in the SWM-SEFPs. HTCs in the SWM-SEFPs is increased by 48% - 89.73% as compared to those in the RMs. The SWMs can reduce the heating film temperatures as compared to the RMs. Furthermore, the temperature oscillations of the heating film and physical mechanisms are discussed according to the flow patterns and image-based analysis. In general, the temperatures of the heating film increase significantly due to the lack of liquid on the inner walls of microchannels and thus significant wall temperature fluctuations occur. The SWM-SEFPs can prevent the backflow and reduce the wall temperature fluctuations.