Ankush Kaundal | Indian Institute of Technology Guwahati (original) (raw)

Papers by Ankush Kaundal

The forces due to surface tension and momentum change during evaporation, in conjunction with the... more The forces due to surface tension and momentum change during evaporation, in conjunction with the forces due to viscous shear and inertia, govern the two-phase flow patterns and the heat transfer characteristics during flow boiling in microchannels. These forces are analyzed in this paper, and two new nondimensional groups, K 1 and K 2 , relevant to flow boiling phenomenon are derived. These groups are able to represent some of the key flow boiling characteristics, including the CHF. In addition, a mechanistic description of the flow boiling phenomenon is presented. The small hydraulic dimensions of microchannel flow passages present a large frictional pressure drop in single-phase and two-phase flows. The small hydraulic diameter also leads to low Reynolds numbers, in the range 100-1000, or even lower for smaller diameter channels. Such low Reynolds numbers are rarely employed during flow boiling in conventional channels. In these low Reynolds number flows, nucleate boiling systematically emerges as the dominant mode of heat transfer. The high degree of wall superheat required to initiate nucleation in microchannels leads to rapid evaporation and flow instabilities, often resulting in flow reversal in multiple parallel channel configuration. Aided by strong evaporation rates, the bubbles nucleating on the wall grow rapidly and fill the entire channel. The contact line between the bubble base and the channel wall surface now becomes the entire perimeter at both ends of the vapor slug. Evaporation occurs at the moving contact line of the expanding vapor slug as well as over the channel wall covered with a thin evaporating film surrounding the vapor core. The usual nucleate boiling heat transfer mechanisms, including liquid film evaporation and transient heat conduction in the liquid adjacent to the contact line region, play an important role. The liquid film under the large vapor slug evaporates completely at downstream locations thus presenting a dryout condition periodically with the passage of each large vapor slug. The experimental data and high speed visual observations confirm some of the key features presented in this paper. Comparison of the Kandlikar †1,2,35 ‡ correlation and Eqs. "15… and "16… for the Nucleate Boiling Dominant Region with Steinke and Kandlikar 36 data in the laminar region, plotted with heat transfer coefficient as a function of local vapor quality x, water, D h Ä207 m, Re LO Ä291, GÄ157 kgÕm 2 s, q Ä473 kWÕm 2

Boiling heat transfer in small diameter tubes has been experimentally investigated using R134a as... more Boiling heat transfer in small diameter tubes has been experimentally investigated using R134a as the working fluid. The heat transfer experiments were conducted with two stainless steel tubes of internal diameter 4.26 and 2.01 mm respectively. Other parameters were varied in the range: mass flux 100-500 kg/ m 2 s; pressure 8-12 bar; quality up to 0.9; heat flux 13-150 kW/m 2 . It was found that the nucleate boiling is dominant when the vapour quality is less than about 40-50% for the 4.26 mm tube and 20-30% for the 2.01 mm tube. Above these quality values, heat transfer coefficients decrease with vapour quality. Furthermore, this decrease occurs for the entire quality range at high heat flux values. Flow visualization experiments were carried out using the same experimental facility with Pyrex glass tubes. A flow pattern map was obtained at a system pressure of 10 bar and tube diameter of 4.26 mm.

The flow boiling heat transfer of refrigerant R134a in horizontal multiport mini-channel was expe... more The flow boiling heat transfer of refrigerant R134a in horizontal multiport mini-channel was experimentally investigated in this paper. Two kinds of mini-channels made of aluminum alloy with the hydraulic diameter of 0.63 mm for 23 rectangular channels and 0.72 mm for 14 rectangular channels were studied. Electric heating film was adopted to provide direct uniform heat flux. The experimental results were obtained with the vapor quality between 0 and 1, the heat flux ranging from 10 to 60 kW/m 2 , the mass flux ranging from 128 to 560 kg/ (m 2 .s) and the saturation pressure from 0.22 to 0.58 MPa. It was found that nucleate boiling dominates in the low vapor quality region and the heat transfer coefficients of mini-channel are higher than those in conventional channel. The heat transfer coefficients increase with the increasing of both heat flux and saturation pressure but have little dependence with the mass flux. In the high vapor quality region, connective boiling is the main contributor. Heat flux and saturation pressure have little influence on the heat transfer coefficients, but the influence of mass flux is obvious. There is a small increase in heat transfer coefficient with an increase of mass flux. Accordingly, a new correlation for flow boiling heat transfer in mini-channel of R134a was proposed based on Gungor and Winterton correlation. The mean relation deviation is −1.76% and the mean absolute relation deviation is 19.0% respectively. This new correlation is proved to have an obvious accuracy improvement in predicting the flow boiling heat transfer coefficient of R134a in mini-channel. The present work can provide a guidance to the design of mini-channel heat exchanger.

An experimental study of R245fa two-phase flow in a 1.6 mm inner diameter circular channel is pre... more An experimental study of R245fa two-phase flow in a 1.6 mm inner diameter circular channel is presented in both adiabatic and diabatic conditions. The test section is composed of a sapphire tube coated with ITO, which enables a total transparency of the evaporator. The effect of inclination on the flow patterns and the pressure drops is presented and discussed for various vapour qualities and mass velocities and a saturation temperature of 81°C, corresponding to a Bond number of 4.2. For each experimental conditions, the pressure drops are measured and the flow is visualised under eleven configurations of inclination from the vertical downward flow (−90°) to the vertical upward flow (+90°). The flow pattern transitions are compared with two flow pattern maps available in the literature. The effect of the heat flux on the flow patterns is analysed and shows the disappearance of the stratified flow for the downward flows in the case of low-inertia flows. A study of the effect of the inclination on the total and frictional pressure gradients is also led. The observations are compared with several models of the literature. These models show a good agreement to predict the pressure gradient for upward flows. Finally, the effect of the heat flux on the pressure drop variations with inclination is analysed. It shows a general increase of the pressure drops due to gravity and frictional forces with the heat flux for low vapour qualities, whatever the considered inclination. This offset decreases with the vapour quality.

A general correlation for heat transfer coefficients of saturated flow boiling is developed based... more A general correlation for heat transfer coefficients of saturated flow boiling is developed based on the first experimental database consisting of 17,778 data points from 101 sources and 13 different fluids. The new correlation makes a significant breakthrough in the prediction accuracy for saturated flow boiling heat transfer, achieving a mean absolute deviation (MAD) of 4.5% against the database, with 68.1% and 89.7% of the data falling within ±5% and ±10% error bands, respectively. The new correlation is validated and compared with 45 existing correlations on the base of the second database of saturated flow boiling heat transfer. The second database contains 6664 data points from 60 sources and 18 different fluids and has no duplicate data from the first. The validation results show that the new correlation has an MAD of 4.4% against the second database, with 69.0% and 93.5% of the data within ±5% and ±10% error bands respectively, further confirming its superior prediction accuracy and reliability. The best existing correlation for the second database, meanwhile, has an MAD of 26.0%, with only 13.0% and 26.0% of the data falling within ±5% and ±10% error bands, respectively. The new correlation is applicable to various channel sizes, flow directions, and flow regimes.

Flow pattern, heat transfer, and pressure drop data for different flow orientations was presented... more Flow pattern, heat transfer, and pressure drop data for different flow orientations was presented in this study. The data was obtained based on flow boiling experiments with R-134a flow through a 1 mm diameter channel which was aligned in different orientations, i.e. horizontal flow, vertical upward flow, and vertical downward flow. A constant surface heat flux condition was performed under a saturation pressure of 8 bar, a heat flux range of 1-60 kW/m 2 , and a mass flux range of 250-820 kg/m 2 s. The experimental results showed the importance of the change in the flow direction. The shape of the gas slug during horizontal flow did not look the same as in the vertical orientations. Heat transfer coefficient and pressure drop became increased when the refrigerant flowed in the vertical downward direction. The experimental data was also compared with the existing prediction methods.

In view of practical significance of a correlation of heat transfer coefficient in the aspects of... more In view of practical significance of a correlation of heat transfer coefficient in the aspects of such applications as engineering design and prediction, some efforts towards correlating flow boiling heat transfer coefficients for mini-channels have been made in this study. Based on analyses of existing experimental investigations of flow boiling, it was found that liquid-laminar and gas-turbulent flow is a common feature in many applications of mini-channels. Traditional heat transfer correlations for saturated flow boiling were developed for liquid-turbulent and gas-turbulent flow conditions and thus may not be suitable in principle to be used to predict heat transfer coefficients in mini-channels when flow conditions are liquid-laminar and gas-turbulent. By considering flow conditions (laminar or turbulent) in the Reynolds number factor F and single-phase heat transfer coefficient h sp , the Chen correlation has been modified to be used for four flow conditions such as liquid-laminar and gas-turbulent one often occurring in mini-channels. A comparison of the newly developed correlation with various existing data for mini-channels shows a satisfactory agreement. In addition, an extensive comparison of existing general correlations with databases for mini-channels has also been made.

This article presents an experimental study of ascendant forced flow boiling in mini-channels wit... more This article presents an experimental study of ascendant forced flow boiling in mini-channels with refrigerant R134a. A flat aluminium multi-port extruded tube composed of 11 parallel rectangular channels (3.28 mm · 1.47 mm) with hydraulic diameter of 2.01 mm was used. Mass flux ranged from 90 to 295 kg/m 2 s and heat flux from 6.0 to 31.6 kW/m 2 . Two working pressures, 405 and 608 kPa, were tested. Inlet subcooling varied from 1 to 17 K. Heat transfer was found to be greater than that previously reported in the literature for conventional tubes, while dry-out occurred at low qualities.

The forces due to surface tension and momentum change during evaporation, in conjunction with the... more The forces due to surface tension and momentum change during evaporation, in conjunction with the forces due to viscous shear and inertia, govern the two-phase flow patterns and the heat transfer characteristics during flow boiling in microchannels. These forces are analyzed in this paper, and two new nondimensional groups, K 1 and K 2 , relevant to flow boiling phenomenon are derived. These groups are able to represent some of the key flow boiling characteristics, including the CHF. In addition, a mechanistic description of the flow boiling phenomenon is presented. The small hydraulic dimensions of microchannel flow passages present a large frictional pressure drop in single-phase and two-phase flows. The small hydraulic diameter also leads to low Reynolds numbers, in the range 100-1000, or even lower for smaller diameter channels. Such low Reynolds numbers are rarely employed during flow boiling in conventional channels. In these low Reynolds number flows, nucleate boiling systematically emerges as the dominant mode of heat transfer. The high degree of wall superheat required to initiate nucleation in microchannels leads to rapid evaporation and flow instabilities, often resulting in flow reversal in multiple parallel channel configuration. Aided by strong evaporation rates, the bubbles nucleating on the wall grow rapidly and fill the entire channel. The contact line between the bubble base and the channel wall surface now becomes the entire perimeter at both ends of the vapor slug. Evaporation occurs at the moving contact line of the expanding vapor slug as well as over the channel wall covered with a thin evaporating film surrounding the vapor core. The usual nucleate boiling heat transfer mechanisms, including liquid film evaporation and transient heat conduction in the liquid adjacent to the contact line region, play an important role. The liquid film under the large vapor slug evaporates completely at downstream locations thus presenting a dryout condition periodically with the passage of each large vapor slug. The experimental data and high speed visual observations confirm some of the key features presented in this paper. Comparison of the Kandlikar †1,2,35 ‡ correlation and Eqs. "15… and "16… for the Nucleate Boiling Dominant Region with Steinke and Kandlikar 36 data in the laminar region, plotted with heat transfer coefficient as a function of local vapor quality x, water, D h Ä207 m, Re LO Ä291, GÄ157 kgÕm 2 s, q Ä473 kWÕm 2

Boiling heat transfer in small diameter tubes has been experimentally investigated using R134a as... more Boiling heat transfer in small diameter tubes has been experimentally investigated using R134a as the working fluid. The heat transfer experiments were conducted with two stainless steel tubes of internal diameter 4.26 and 2.01 mm respectively. Other parameters were varied in the range: mass flux 100-500 kg/ m 2 s; pressure 8-12 bar; quality up to 0.9; heat flux 13-150 kW/m 2 . It was found that the nucleate boiling is dominant when the vapour quality is less than about 40-50% for the 4.26 mm tube and 20-30% for the 2.01 mm tube. Above these quality values, heat transfer coefficients decrease with vapour quality. Furthermore, this decrease occurs for the entire quality range at high heat flux values. Flow visualization experiments were carried out using the same experimental facility with Pyrex glass tubes. A flow pattern map was obtained at a system pressure of 10 bar and tube diameter of 4.26 mm.

The flow boiling heat transfer of refrigerant R134a in horizontal multiport mini-channel was expe... more The flow boiling heat transfer of refrigerant R134a in horizontal multiport mini-channel was experimentally investigated in this paper. Two kinds of mini-channels made of aluminum alloy with the hydraulic diameter of 0.63 mm for 23 rectangular channels and 0.72 mm for 14 rectangular channels were studied. Electric heating film was adopted to provide direct uniform heat flux. The experimental results were obtained with the vapor quality between 0 and 1, the heat flux ranging from 10 to 60 kW/m 2 , the mass flux ranging from 128 to 560 kg/ (m 2 .s) and the saturation pressure from 0.22 to 0.58 MPa. It was found that nucleate boiling dominates in the low vapor quality region and the heat transfer coefficients of mini-channel are higher than those in conventional channel. The heat transfer coefficients increase with the increasing of both heat flux and saturation pressure but have little dependence with the mass flux. In the high vapor quality region, connective boiling is the main contributor. Heat flux and saturation pressure have little influence on the heat transfer coefficients, but the influence of mass flux is obvious. There is a small increase in heat transfer coefficient with an increase of mass flux. Accordingly, a new correlation for flow boiling heat transfer in mini-channel of R134a was proposed based on Gungor and Winterton correlation. The mean relation deviation is −1.76% and the mean absolute relation deviation is 19.0% respectively. This new correlation is proved to have an obvious accuracy improvement in predicting the flow boiling heat transfer coefficient of R134a in mini-channel. The present work can provide a guidance to the design of mini-channel heat exchanger.

An experimental study of R245fa two-phase flow in a 1.6 mm inner diameter circular channel is pre... more An experimental study of R245fa two-phase flow in a 1.6 mm inner diameter circular channel is presented in both adiabatic and diabatic conditions. The test section is composed of a sapphire tube coated with ITO, which enables a total transparency of the evaporator. The effect of inclination on the flow patterns and the pressure drops is presented and discussed for various vapour qualities and mass velocities and a saturation temperature of 81°C, corresponding to a Bond number of 4.2. For each experimental conditions, the pressure drops are measured and the flow is visualised under eleven configurations of inclination from the vertical downward flow (−90°) to the vertical upward flow (+90°). The flow pattern transitions are compared with two flow pattern maps available in the literature. The effect of the heat flux on the flow patterns is analysed and shows the disappearance of the stratified flow for the downward flows in the case of low-inertia flows. A study of the effect of the inclination on the total and frictional pressure gradients is also led. The observations are compared with several models of the literature. These models show a good agreement to predict the pressure gradient for upward flows. Finally, the effect of the heat flux on the pressure drop variations with inclination is analysed. It shows a general increase of the pressure drops due to gravity and frictional forces with the heat flux for low vapour qualities, whatever the considered inclination. This offset decreases with the vapour quality.

A general correlation for heat transfer coefficients of saturated flow boiling is developed based... more A general correlation for heat transfer coefficients of saturated flow boiling is developed based on the first experimental database consisting of 17,778 data points from 101 sources and 13 different fluids. The new correlation makes a significant breakthrough in the prediction accuracy for saturated flow boiling heat transfer, achieving a mean absolute deviation (MAD) of 4.5% against the database, with 68.1% and 89.7% of the data falling within ±5% and ±10% error bands, respectively. The new correlation is validated and compared with 45 existing correlations on the base of the second database of saturated flow boiling heat transfer. The second database contains 6664 data points from 60 sources and 18 different fluids and has no duplicate data from the first. The validation results show that the new correlation has an MAD of 4.4% against the second database, with 69.0% and 93.5% of the data within ±5% and ±10% error bands respectively, further confirming its superior prediction accuracy and reliability. The best existing correlation for the second database, meanwhile, has an MAD of 26.0%, with only 13.0% and 26.0% of the data falling within ±5% and ±10% error bands, respectively. The new correlation is applicable to various channel sizes, flow directions, and flow regimes.

Flow pattern, heat transfer, and pressure drop data for different flow orientations was presented... more Flow pattern, heat transfer, and pressure drop data for different flow orientations was presented in this study. The data was obtained based on flow boiling experiments with R-134a flow through a 1 mm diameter channel which was aligned in different orientations, i.e. horizontal flow, vertical upward flow, and vertical downward flow. A constant surface heat flux condition was performed under a saturation pressure of 8 bar, a heat flux range of 1-60 kW/m 2 , and a mass flux range of 250-820 kg/m 2 s. The experimental results showed the importance of the change in the flow direction. The shape of the gas slug during horizontal flow did not look the same as in the vertical orientations. Heat transfer coefficient and pressure drop became increased when the refrigerant flowed in the vertical downward direction. The experimental data was also compared with the existing prediction methods.

In view of practical significance of a correlation of heat transfer coefficient in the aspects of... more In view of practical significance of a correlation of heat transfer coefficient in the aspects of such applications as engineering design and prediction, some efforts towards correlating flow boiling heat transfer coefficients for mini-channels have been made in this study. Based on analyses of existing experimental investigations of flow boiling, it was found that liquid-laminar and gas-turbulent flow is a common feature in many applications of mini-channels. Traditional heat transfer correlations for saturated flow boiling were developed for liquid-turbulent and gas-turbulent flow conditions and thus may not be suitable in principle to be used to predict heat transfer coefficients in mini-channels when flow conditions are liquid-laminar and gas-turbulent. By considering flow conditions (laminar or turbulent) in the Reynolds number factor F and single-phase heat transfer coefficient h sp , the Chen correlation has been modified to be used for four flow conditions such as liquid-laminar and gas-turbulent one often occurring in mini-channels. A comparison of the newly developed correlation with various existing data for mini-channels shows a satisfactory agreement. In addition, an extensive comparison of existing general correlations with databases for mini-channels has also been made.

This article presents an experimental study of ascendant forced flow boiling in mini-channels wit... more This article presents an experimental study of ascendant forced flow boiling in mini-channels with refrigerant R134a. A flat aluminium multi-port extruded tube composed of 11 parallel rectangular channels (3.28 mm · 1.47 mm) with hydraulic diameter of 2.01 mm was used. Mass flux ranged from 90 to 295 kg/m 2 s and heat flux from 6.0 to 31.6 kW/m 2 . Two working pressures, 405 and 608 kPa, were tested. Inlet subcooling varied from 1 to 17 K. Heat transfer was found to be greater than that previously reported in the literature for conventional tubes, while dry-out occurred at low qualities.