Controlled Flow Evaporation in Complex Microchannels With Non-Uniform Wall Temperatures

Abstract

This paper presents an experimental study of flow evaporation in non-uniform microchannels, demonstrating the ability to provide a stable flow of evaporated fluid for energy conversion and chip cooling applications. Two mechanisms are proposed to stabilize the internal flow evaporation. The first mechanism is to establish a temperature gradient along the channel to separate the room temperature inlet fluid from the steam exit flow. The second mechanism is to change the direction of the surface tension forces acting on the meniscus to fix its position along the channel. To achieve this, shaped channels are formed of contractions and expansions with varied wall angles. The device consists of a silicon wafer with through-etched complex microchannels, that is anodically bonded to a glass wafer on each side. Inlet and exit holes for the fluid are machined in the glass wafers. Water is forced through the chip while it is heated on the exit side of the three layer chip. The qualitative nature of the two-phase flow along the shaped channels is observed through the glass cover wafer, for different flow rates and wall temperatures. The temperature gradient achieved with different thickness of channel walls shows agreement with the modeling results. Also, the benefit of having multiple expansions in the channels was demonstrated. By using these two mechanisms the onset of water evaporation was fixed along the channel. This will lead to the development of adequate two-phase flow micro heat exchangers.

Experimental and numerical investigation of a shaped microchannel evaporator for a micro Rankine cycle application

“…6 (without the top bonded glass layer). Details of fabrication process were provided by Arslan [21,22]. Fig.…”

“…Otherwise, the cell fills with water and it flows to the next cell with higher potential of evaporation because of its higher wall temperature. Arslan et al [21,22] showed that a stable evaporating meniscus can be observed at different locations along the shaped channel, depending on the water mass flow rate and the temperature gradient along the channel. Boiling and the formation of detrimental vapor bubbles within the microchannel were prevented and a fully evaporated flow of superheated steam was provided at the exit.…”

“…Fully evaporated steam flow in a microchannel was achieved by Arslan et al [21,22] using a thermal gradient along shaped channels with multiple expansions and contractions. The schematic of a shaped microchannel evaporator is shown in Fig.…”

Modeling the Performance of Bi-Textured Micropillar Array as a Wicked Evaporator

“…They have been proposed and studied mainly for cooling of advanced semiconductor devices [1]. Another application for thin film evaporation is to provide a controllable flow of fully evaporated steam for micro chemical processing and micro heat engines [2,3]. One approach consists of conducting the phase change in microchannels, but it has often lead to inconsistent vapor quality at the exit.…”

“…One approach consists of conducting the phase change in microchannels, but it has often lead to inconsistent vapor quality at the exit. Alternatively, shaped microchannels have been shown to provide fully evaporate steam, using surface tension and a streamwise temperature gradient [2,3]. Experimental results showed that a limited range of evaporation mass flow rate can be achieved with this method due to its limited liquid thin film area.…”