Boiling Heat Transfer in Minichannels

Abstract

the capillary forces predominate, and they determine flow pattern and boiling heat transfer. The interest for study of boiling heat transfer in minichannels increases at last decade due to obtained possibility to makes the channels of millimeter’s and micron’s size for application in wide range of industrial devices for enhancement of heat and mass transfer. They could be used in compact evaporator/condenser of cryogenic devices in form of vacuum brazed fin passage. Another area of application of microchannel architecture is micro-heat exchangers and micro-cooling assemblies referred to as micro-thermal-mechanical systems (MTMS) used as micro-cooling elements. They also could be used as micro-chemical reactors, which operate with a small residence time. The characteristic of mini/microchannel assemblies is high channel density and large surface area, which considerably increases the rate of heat transfer. When boiling/condensing occurs inside of very small and non-circular passages the capillary forces become important in determining the aspects of flow phenomena such as flow pattern, shape of interface and macroscale heat transfer. In the literature the transition between large scale and mini scale channels is not exactly defined. At paper [1], the next channel size classification on its hydraulic diameter is proposed: microchannels (size of 1 to 100Pm), mesochannels (size of 100Pm to 1mm), macrochannels (size of 1 to 6 mm) and conventional channels (size more than 6 mm). In contrarily in [2] proposed another channel size classification on its hydraulic diameter: microchannels (size of 50 to 600Pm), minichannels (size of 600Pm to 3 mm) and conventional channels (size more than 3 mm). In current lecture, we will follow classification [2] and the channels with size of 600Pm to 3 mm will be under consideration. There are many papers where the boiling heat transfer was analyzed for such channels. In contrast to boiling in conventional tubes the flow boiling heat transfer coefficients in microchannels are dependent on heat flux and pressure while only slightly dependent on flow velocity and vapor quality [3], [4], [5], [6], [7], [8]. Many experimental study have concluded that for wall superheat the nucleate boiling is dominant mechanism for evaporation in minichannels with a small convective evaporation contribution [3], [4], [5], [6], [7], [9]. Another studies concluded that for their tests with multichannel arrangement the nucleation is not important mechanism [10], [11], [12], [13]. The weak dependence of heat transfer coefficient on heat flux density was observed for boiling of low thermal conductivity liquid in vertical slot with boiling induced liquid circulation [14]. It was shown also that the heat transfer to be dependent on the existing flow pattern [8], [15]. At paper [16] suggested that the transient evaporation of thin liquid film surrounding elongated bubbles is the dominant heat transfer mechanism, not nucleate boiling. It was concluded that macroscale models are not realistic for predicting flow boiling heat transfer in mini/microchannels since they are based on the nucleate boiling and convective evaporation mechanism and not accounts the effect of surface tension [2], [6], [8], [17], [18]. 255