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Nucleation and bubble dynamics on a heater surface contribute to high heat transfer rate in pool boiling. Introducing two-phase flow in narrow channels further improves heat transfer. Use of expanding taper microgap geometry further enhances heat transfer, and proper balancing of taper angles and flow lengths leads to self-sustained flow boiling in tapered microgap geometries. This paper focuses on understanding the underlying enhancement mechanism by studying the bubble behavior as they expand and accelerate in the direction of increased taper. The present study conducts a 2D simulation analysis of bubble growth in tapered microgaps with numerical simulations to identify the effect of the fluid properties and tapered angle in the bubble and fluid dynamics behavior. Ansys-Fluent is customized with user-defined-functions (UDFs) accounting for the interfacial heat and mass transport, including a sharp interface and direct calculation of mass transfer with temperature gradients. The study was conducted using air injection and boiling simulation from the conception to the departure of a bubble. The tapered angles were 5°, 10°, and 15°, with flowrates between 3 ml/min to 30 ml/min, 1 mm air inlet, and at 1 mm distance from the convergent end. The departure time of 10 subsequent bubbles was recorded to check the configuration with the quickest bubble removal. A critical flowrate and surface tension region was established for the escape direction of the bubble. In addition, the numerical simulation considered the tapered microgap with a nucleating bubble at atmospheric conditions with a wall superheats of 5 K. The results show that the bubble growing over the heated surface creates fluid circulations and interfacial conditions that suppress the thermal boundary layer leading to an increased local heat transfer coefficient within a range of 1 mm from the interface. 

Heat transfer due to the convective boiling mechanism in the microchannel plays an important role in heat transfer during boiling. Therefore, it is relevant to find ways to manipulate the vapor bubbles such that convection heat transfer is enhanced. This numerical study investigates the effects of different geometrical parameters on bubble movement through a micro tapered gap. The objective is to identify an optimal configuration such that the bubble moves at the fastest possible speed when it travels through the micro gap. To conduct this research a model is created using ANSYS-Fluent which uses the Volume of Fluid (VOF) interface tracking method. The multiphase VOF model tracks the air-water interface. A bubble is generated inside the microchannel in which fluid is flowing. The overall domain of the model consists of the surface at the bottom, having an orifice through which the air bubble is generated. Three different cases of an angled tapered surface are created 5°, 10°, and 15°. The airflow rate is kept constant throughout each simulation. Simulation results show the impact of the tapered angle on the bubble’s flow movement and flow direction. Liquid and air velocity contours can be used to analyze the flow. The impact of the taper angles on the movement and flow direction of the air bubble is discussed. It is observed that the performed simulations help to better understand the experimental observation of bubble motion; the simulations give clear evidence of the fluid dynamic behavior along the tapered microchannel.