Two phase flows in mini channels occur in many industrial applications such as electronic cooling, compact heat exchangers, compact refrigeration systems and in micro propulsion devices. Due to its significance, research on two phase flow in mini channels has become attractive. However, in recent times a controversy exists whether flow in minichannel is different from macro flow because there are still substantial disagreements among various experimental results. In the present study an experimental investigation is carried out for fluid flow and boiling heat transfer characteristics of mini channels with tube diameters ranging from 1-3mm. The tubes were made of SS with water as the working fluid. The variation in friction factor and Nusselt number with decrease in tube diameter for single phase flow was systematically studied. The point of Onset of Nucelate Boiling (ONB) was identified based on wall temperature profile. The effect of heat flux and mass flux on two phase pressure drop with three different tube diameters during sub cooled boiling were investigated. The results reveal that there is an unmistakable effect of tube diameter on fluid friction and onset of boiling during sub cooled boiling in tubes of mini channel dimensions. © 2010 by ASME.

Sarit Kumar Das

Department of Mechanical Engineering

Manoharan Aravinthan

Arcot R. Balakrishnan

Boiling heat transfer

Compact heat exchanger

Experimental investigations

Fluid friction

Friction factors

MICRO PROPULSION

MINI CHANNELS

Refrigeration system

Single-phase flow

Sub-cooled boiling

Tube diameters

Two-phase pressure drops

Wall temperatures

Working fluid

Flow of fluids

Heat flux

Industrial applications

Nusselt number

Tubes (components)

Two phase flow

Phase Transitions

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Fluid Flow and Boiling Heat Transfer in Mini Channels

2010 14th International Heat Transfer Conference, IHTC 14

The effect of tube diameter on two-phase frictional pressure drop was investigated in circular tubes with inner diameters of 0.6, 1.2, 1.7, 2.6 and 3.4. mm using air and water. The gas and liquid superficial velocity ranges were 0.01-50. m/s and 0.01-3. m/s, respectively. The gas and liquid flow rates were measured and the two-phase flow pattern images were recorded using high-speed CMOS camera. Unique flow patterns were observed for smaller tube diameters. Pressure drop was measured and compared with various existing models such as homogeneous model and Lockhart-Martinelli model. It appears that the dominant effect of surface tension shrinking the flow stratification in the annular regime is important. It was found that existing models are inadequate in predicting the pressure drop for all the flow regimes visualized. Based on the analysis of present experimental frictional pressure drop data a correlation is proposed for predicting Chisholm parameter " C" in slug annular flow pattern. For all other flow regimes Chisholm's original correlation appears to be adequate except the bubbly flow regime where homogeneous model works well. The modification results in overall mean deviation of pressure drop within 25% for all tube diameters considered. This approach of flow regime based modification of liquid gas interaction parameter appears to be the key to pressure drop prediction in narrow tubes. © 2010 Elsevier Inc.

Experimental Thermal and Fluid Science

Effect of diameter on two-phase pressure drop in narrow tubes

The effect of superficial velocity of liquid and gas on elongated bubble length is experimentally studied using water and air as the working fluid inside mini channels of tube diameters less than 3mm. A high speed CMOS camera is used to identify the slug length. The reduction in tube diameter results in squeezing of bubbles and thereby results in elongated bubble flow pattern distinguishing it from slug flow pattern. At low superficial gas velocities inside mini/micro channels, such pattern dominates which is the subject of interest in the present study. The experimental set-up used in this study is designed for adiabatic co-current flow of air-water mixtures in round horizontal tubes. Both the liquid and gas streams flowed separately through a bank of rota meters and Coriolis mass flow meters before entering the gas-liquid mixer. The gas-liquid mixer installed upstream of the test section (the observation zone), ensures that the two phases get thoroughly mixed before entering the test section. Flow patterns were observed and the video graphs were recorded in a computer and subsequently analyzed frame by frame with appropriate image processing technique to determine various flow regimes. The decreasing tube diameter results in the elongated bubble flow pattern over a wider range in the flow regime map of mini channels which would be of a slug flow pattern in conventional tube diameters. The region of slug flow is differentiated from elongated bubble flow based on flow visualizations. The analysis of measured length of elongated bubble acts as a good indicator for identifying the change of flow regime in mini channels. © 2010 IEEE.

2010 3rd International Conference on Thermal Issues in Emerging Technologies, Theory and Applications - Proceedings, ThETA3 2010

Effect of tube diameter on elongated bubble length in mini channels

The effect of tube diameter on two-phase flow patterns was investigated in circular tubes with inner diameters of 0.6, 1.2, 1.7, 2.6, and 3.4 mm using air and water. The gas and liquid superficial velocity ranges were 0.01-50 and 0.01-3 m/s, respectively. The gas and liquid flow rates were measured and the two-phase flow pattern images were recorded using high-speed CMOS camera. The flow patterns observed were dispersed bubbly, bubbly, slug, slug-annular, wavy-annular, stratified, and annular flows. These flow patterns were not observed in all the test diameters, but were found to be unique to particular tube diameters, confirming the effect of tube diameter on the flow pattern. The data obtained were compared to existing experimental data and flow regime transition maps which show generally reasonable overall agreement at the larger diameters, but significant differences were observed with the smaller diameter tubes. © 2010 Canadian Society for Chemical Engineering.

Canadian Journal of Chemical Engineering

Effect of tube diameter on two-phase flow patterns in mini tubes

International Journal of Heat and Fluid Flow

Review on two-phase flow instabilities in narrow spaces

Sintered porous medium heat sink for cooling of high-power mini-devices

Energy Conversion and Management

Static and dynamic flow instability of a parallel microchannel heat sink at high heat fluxes

Nuclear Engineering and Design

Pressure drops for steam and water flow in heated tubes

Fluid flow and boiling heat transfer in mini channels

Journal	2010 14th International Heat Transfer Conference, IHTC 14
Open Access	No