Two-phase pressure drop with twisted-tape swirl generators

An experimental study has been conducted to determine the effect of twisted-tape swirl generators on adiabatic and diabatic two-phase flow pressure drops in vertical straight tubes. Tape-twist ratios (length for 180° twist/inside tube diameter) of 3.94, 8.94, and 13.92 were tested with R-113 over a range of pressures, mass velocities, qualities, and heat fluxes. Empty tube refcrence data were successfully predicted with a correlation from the literature. The twisted tape data were successfully correlated by using the hydraulic diameter and a single-phase swirl flow friction factor in the empty tube correlation. Data from the literature also were predicted well with this correlation.     

Flow boiling heat transfer characteristics of R123 and R134a in a micro-channel

Flow boiling heat transfer in a single circular micro-channel of 0.19 mm ID has been experimentally investigated with R123 and R134a for various experimental conditions: heat fluxes (10, 15, 20 kW/m²), mass velocities (314, 392, 470 kg/m² s), vapor qualities (0.2–0.85) and different saturation pressures (158, 208 kPa for R123; 900, 1100 kPa for R134a). The heat transfer trends between R123 and R134a are clearly distinguished. Whether nucleate boiling is suppressed at low vapor quality or not determines the heat transfer trend and mechanism in the flow boiling of micro-channels. High convective heat transfer coefficients in the two-phase flow of micro-channels enables nucleate boiling to be suppressed even at low vapor quality, depending on the wall superheat requirement for nucleate boiling. In the case of early suppression of nucleate boiling, specifically R123, heat transfer is dominated by evaporation of thin liquid films around elongated bubbles. In the contrary case, namely R134a, nucleate boiling is dominant heat transfer mechanism until its suppression at high vapor quality and then two-phase forced convection heat transfer becomes dominant. It is similar to the heat transfer characteristic of macro-channels except the enhancement of nucleate boiling and the short forced convection region.     

Pressure drop caused by flow area changes in capillaries under low flow conditions

Single-phase and two-phase flow pressure drops caused by flow area expansion and contraction were measured using air and water. The test section consisted of two capillaries with 0.84 mm and 1.6 mm diameters. For single-phase flow, the Reynolds numbers defined based on the smaller diameter capillary covered the range 160–11,000. For two-phase flow, the all-liquid Reynolds number based on the smaller capillary varied in the 410–1020 range, and the flow quality varied in the 0.018–0.2 range. The single-phase flow loss coefficients for both flow area expansion and contraction were empirically correlated. For two-phase flow, the data indicated the occurrence of significant velocity slip, and the one-dimensional homogeneous flow model utterly disagreed with the data. For flow area expansion the one-dimensional slip flow model along with an Armand-type slip ratio correlation could predict the data well. For flow area contraction, the one-dimensional slip flow model along with the slip ratio expression of Zivi agreed with the data very well, provided that no vena-contracta was considered.     

Experimental investigation of flow boiling characteristics in a cross-linked microchannel heat sink

This paper experimentally investigates flow boiling characteristics in a cross-linked microchannel heat sink at low mass fluxes and high heat fluxes. The heat sink consists of 45 straight microchannels each with a hydraulic diameter of 248 μm and heated length of 16 mm. Three cross-links, of width 500 μm, are introduced in the present microchannel heat sink to achieve better temperature uniformity and to avoid flow mal-distribution. Flow visualization, flow instability, two-phase pressure drop, and two-phase heat transfer measurements are conducted using the dielectric coolant FC-72 over a range of heat flux from 7.2 to 104.2 kW/m², mass flux from 99 to 290 kg/m² s, and exit quality from 0.01 to 0.71. Thermochromic liquid crystals are used in the present study as full-field surface temperature sensors to map the temperature distribution on the heat sink surface. Flow visualization studies indicate that the observed flow regime is primarily slug. Visual observations of flow patterns in the cross-links demonstrate that bubbles nucleate and grow rapidly on the surface of the cross-links and in the tangential direction at the microchannels’ entrance due to the effect of circulations generated in those regions. The two-phase pressure drop strongly increases with the exit quality, at x_e,o < 0.3, and the two-phase frictional pressure drop increases by a factor of 1.6–2 compared to the straight microchannel heat sink. The flow boiling heat transfer coefficient increases with increasing exit quality at a constant mass flux, which is caused by the dominance of the nucleation boiling mechanism in the cross-link region.     

Adiabatic and diabatic two-phase venting flow in a microchannel

The growth and advection of the vapor phase in two-phase microchannel heat exchangers increase the system pressure and cause flow instabilities. One solution is to locally vent the vapor formed by capping the microchannels with a porous, hydrophobic membrane. In this paper we visualize this venting process in a single 124 μm by 98 μm copper microchannel with a 65 μm thick, 220 nm pore diameter hydrophobic Teflon membrane wall to determine the impact of varying flow conditions on the flow structures and venting process during adiabatic and diabatic operation. We characterize liquid velocities of 0.14, 0.36 and 0.65 m/s with superficial air velocities varying from 0.3 to 8 m/s. Wavy-stratified and stratified flow dominated low liquid velocities while annular type flows dominated at the higher velocities. Gas/vapor venting can be improved by increasing the venting area, increasing the trans-membrane pressure or using thinner, high permeability membranes. Diabatic experiments with mass flux velocities of 140 and 340 kg/s/m² and exit qualities up to 20% found that stratified type flows dominate at lower mass fluxes while churn-annular flow became more prevalent at the higher mass-flux and quality. The diabatic flow regimes are believed to significantly influence the pressure-drop and heat transfer coefficient in vapor venting heat exchangers.     

Boiling heat transfer,pressure drop,and flow pattern in a horizontal square minichannel

This study experimentally investigated the flow boiling heat transfer, pressure drop, and flow pattern in a horizontal square minichannel with a hydraulic diameter of 2.0 mm, and the effects of mass flux, vapor quality, heat flux, and refrigerant properties on the flow boiling characteristics were clarified. The heat transfer coefficient and pressure drop of R32 and R1234yf were measured in a mass flux range of 50–400 kgm⁻²s⁻¹ at a saturation temperature of 15 °C. The flow pattern of the square minichannel outlet was observed and was classified as plug, wavy, churn, and annular flows. The heat transfer coefficients in the square minichannel were larger than those in the circular minichannel with a similar hydraulic diameter at low mass flux conditions. The heat transfer coefficients of R32 indicated higher values compared with those of R1234yf at same mass flux and qualities. An empirical heat transfer model taking into account the forced convection, nucleate boiling, and thin liquid film evaporation was developed for horizontal square and circular minichannels. The frictional pressure drop of R32 was 1.5–2 times higher than that of R1234yf at same mass flux and vapor quality condition, and the effect of channel shape on the frictional pressure drop was small unlike the boiling heat transfer.     

Explosive boiling of water in parallel micro-channels

The objective of this study is to visualize the flow pattern and to measure heat transfer coefficient during explosive boiling of water in parallel triangular micro-channels. Tests were performed in the range of inlet Reynolds number 25–60, mass flux 95–340 kg/m²s, and heat flux 80–330 kW/m².The flow visualization showed that the behavior of long vapor bubbles, occurring in a micro-channel at low Reynolds numbers, was not similar to annular flow with interposed intermitted slugs of liquid between two long vapor trains. This process may be regarded as explosive boiling with periodic wetting and dryout.In the presence of two-phase liquid–vapor flow in the micro-channel, there are pressure drop oscillations, which increase with increasing vapor quality.This study shows strong dependence of the heat transfer coefficient on the vapor quality. The time when liquid wets the heated surface decreases with increasing heat flux. Dryout occurs immediately after venting of the elongated bubble.     

Flow boiling heat transfer in a vertical spirally internally ribbed tube

 Experiments of flow boiling heat transfer and two-phase flow frictional pressure drop in a spirally internally ribbed tube (φ22×5.5 mm) and a smooth tube (φ19×2 mm) were conducted, respectively, under the condition of 6×10⁵ Pa (absolute atmosphere pressure). The available heated length of the test sections was 2500 mm. The mass fluxes were selected, respectively, at 410, 610 and 810 kg/m² s. The maximum heat flux was controlled according to exit quality, which was no more than 0.3 in each test run. The experimental results in the spirally internally ribbed tube were compared with that in the smooth tube. It shows that flow boiling heat transfer coefficients in the spirally internally ribbed tube are 1.4–2 times that in the smooth tube, and the flow boiling heat transfer under the condition of smaller temperature differences can be achieved in the spirally internally ribbed tube. Also, the two-phase flow frictional pressure drop in the spirally internally ribbed tube increases a factor of 1.6–2 as compared with that in the smooth tube. The effects of mass flux and pressure on the flow boiling heat transfer were presented. The effect of diameters on flow boiling heat transfer in smooth tubes was analyzed. Based on the fits of the experimental data, correlations of flow boiling heat transfer coefficient and two-phase flow frictional factor were proposed, respectively. The mechanisms of enhanced flow boiling heat transfer in the spirally internally ribbed tube were analyzed. Received on 1 December 1999     

Pool boiling heat transfer of ultra-light copper foam with open cells

High speed visualizations and thermal performance studies of pool boiling heat transfer on copper foam covers were performed at atmospheric pressure, with the heating surface area of 12.0 mm by 12.0 mm, using acetone as the working fluid. The foam covers have ppi (pores per inch) from 30 to 90, cover thickness from 2.0 to 5.0 mm, and porosity of 0.88 and 0.95. The surface superheats are from −20 to 190 K, and the heat fluxes reach 140 W/cm². The 30 and 60 ppi foam covers show the periodic single bubble generation and departure pattern at low surface superheats. With continuous increases in surface superheats, they show the periodic bubble coalescence and/or re-coalescence pattern. Cage bubbles were observed to be those with liquid filled inside and vented to the pool liquid. For the 90 ppi foam covers, the bubble coalescence takes place at low surface superheats. At moderate or large surface superheats, vapor fragments continuously escape to the pool liquid.     

Heat transfer characteristics of water and APG surfactant solution in a micro-channel heat sink

The steady increase in internal heat production of cost and high performance electronic components has lead researchers to seek improved ways to remove the heat generated. Single-phase liquid flow has been considered as a potential solution for solving this cooling problem. However, when considering that any solution needs to be of low cost and low mass fluxes and yet retain low temperature gradients across the electronic components, it seems that two-phase boiling flow is preferred. Surfactant solutions have been introduced in connection with enhancement of the boiling processes. We investigated the effects of surfactant solution flows through a micro-channel heat sink. The experimental setup included a high-speed IR radiometer and a CCD camera that were used to characterize the test module. The module consisted of inlet and outlet manifolds that distributed surfactant solutions through an array of 26 parallel micro-channels. The experimental results have shown that there exists an optimal solution concentration and mass flux for enhancing heat removal. Surfactant solution boiling flows were also found to stabilize the maximum and average surface temperatures for a wide range of applied heat fluxes. In addition, the use of surfactant solutions at low mass fluxes has led to CHF enhancement when compared to regular water flows. In the last part of this work, possible explanations for the observed non-ionic surfactant effects are presented.     

Macro-to-microchannel transition in two-phase flow: Part 2 - Flow boiling heat transfer and critical heat flux

This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co ≈ 0.99) and 1.03 (Co ≈ 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1].     

Effects of dynamic load on flow and heat transfer of two-phase boiling water in a horizontal pipe

An experimental investigation was performed to obtain the flow and heat transfer characteristics of single-phase water flow and two-phase pipe boiling water flow under high gravity (Hi-G) in present work. The experiments were conducted on a rotating platform, and boiling two-phase flow state was obtained by means of electric heating. The data were collected specifically in the test section, which was a lucite pipe with inner diameter of 20 mm and length of 400 mm. By changing the parameters, such as rotation speed, inlet temperature, flow rate, and etc., and analyzing the fluid resistance, effective heat and heat transfer coefficient of the experimental data, the effects of dynamic load on the flow and heat transfer characteristics of single phase water and two-phase boiling water flow were investigated and obtained. The two-phase flow patterns under Hi-G condition were obtained with a video camera. The results show that the dynamic load significantly influences the flow characteristic and boiling heat transfer of the two-phase pipe flow. As the direction of the dynamic load and the flow direction are opposite, the greater the dynamic load, the higher the outlet pressure and the flow resistance, and the lower the flow rate, the void fraction, the wall inner surface temperature and the heat transfer capability. Therefore, the dynamic load will block the fluid flow, enhance heat dissipation toward the ambient environment and reduce the heat transfer to the two-phase boiling flow.     

Volume of fluid approach of boiling flows in concentrated solar plants

The Computational Fluid Dynamics (CFD) model proposed in this paper allows the flow patterns that evolve during progressive boiling inside large scale horizontal tubes to be simulated from the initial vapor generation stage to large vapor slugs. The volume of fluid (VOF) model was employed in combination with relatively simple hypothesis. The aim of the present work is to improve the design of receiver tubes at concentrated solar power plants with direct steam generation by simulating the evolution of flow regimes within these tubes. Despite numerous studies conducted in the past years on convective boiling, only a few made use of the VOF model to simulate large flow regime transitions. This work presents a preliminary and relatively qualitative approach to address this problem. Heat and mass transfer at the tube inner wall and at the liquid-gas interface were solved with the additional transport of two scalars. One accounts for the enthalpy field and the other represents the dispersed vapor phase of the liquid. This new phase was created at the wall surface of the liquid phase and rises up to the liquid-vapor interface. Different phenomena linked to the boiling process were taken into account: vapor creation at the wall, its transport, recondensation and the creation of large structures. This model was validated with boiling flow in a bent tube at different mass flow rates and heat fluxes, which allowed us to observe the evolution of two-phase flow patterns. Finally, numerical simulation of direct steam generation inside a concentrated solar plant receiver clearly showed the apparition and evolution of various two-phase flow patterns.     

Carbon dioxide flow boiling in a single microchannel - Part I: Pressure drops

Carbon dioxide two-phase flow pressure drops have been investigated in a single horizontal stainless-steel micro-tube having a 0.529 mm inner diameter. Experiments were carried out in adiabatic conditions for four saturation temperatures of −10; −5; 0; 5 °C and mass fluxes ranging from 200 to 1400 kg/m² s, for inlet qualities up to unity. Measurements have been compared to the predictions of well-known methods. The Müller-Steinhagen and Heck correlation and the Friedel correlation gave the best fit as well as the homogeneous model when the liquid viscosity is used to represent the apparent two-phase viscosity. Further, an analysis based on the homogeneous model has not shown any clear appearance of the laminar or the transition regimes in any given range of Reynolds number. The apparent viscosity of the two-phase mixture was found larger than the liquid viscosity at low vapour qualities, namely at the lowest temperatures. Hence, a new expression to determine the equivalent viscosity was suggested as a function of the reduced pressure. Lastly, the Chisholm parameter from the Lockhart-Martinelli correlation was found lower than expected and also mainly dependent on the saturation temperature.     

Two-phase pressure drop and flow visualization of FC-72 in a silicon microchannel heat sink

The rapid development of two-phase microfluidic devices has triggered the demand for a detailed understanding of the flow characteristics inside microchannel heat sinks to advance the cooling process of micro-electronics. The present study focuses on the experimental investigation of pressure drop characteristics and flow visualization of a two-phase flow in a silicon microchannel heat sink. The microchannel heat sink consists of a rectangular silicon chip in which 45 rectangular microchannels were chemically etched with a depth of 276 μm, width of 225 μm, and a length of 16 mm. Experiments are carried out for mass fluxes ranging from 341 to 531 kg/m² s and heat fluxes from 60.4 to 130.6 kW/m² using FC-72 as the working fluid. Bubble growth and flow regimes are observed using high speed visualization. Three major flow regimes are identified: bubbly, slug, and annular. The frictional two-phase pressure drop increases with exit quality for a constant mass flux. An assessment of various pressure drop correlations reported in the literature is conducted for validation. A new general correlation is developed to predict the two-phase pressure drop in microchannel heat sinks for five different refrigerants. The experimental pressure drops for laminar-liquid laminar-vapor and laminar-liquid turbulent-vapor flow conditions are predicted by the new correlation with mean absolute errors of 10.4% and 14.5%, respectively.     

Freon R141b flow boiling in silicon microchannel heat sinks: experimental investigation

This paper presents experimental investigations on Freon R141b flow boiling in rectangular microchannel heat sinks. The main aim is to provide an appropriate working fluid for microchannel flow boiling to meet the cooling demand of high power electronic devices. The microchannel heat sink used in this work contains 50 parallel channels, with a 60 × 200 (W × H) μm cross-section. The flow boiling heat transfer experiments are performed with R141b over mass velocities ranging from 400 to 980 kg/(m² s) and heat flux from 40 to 700 kW/m², and the outlet pressure satisfying the atmospheric condition. The fluid flow-rate, fluid inlet/outlet temperature, wall temperature, and pressure drop are measured. The results indicate that the mean heat transfer coefficient of R141b flow boiling in present microchannel heat sinks depends heavily on mass velocity and heat flux and can be predicted by Kandlikar’s correlation (Heat Transf Eng 25(3):86–93, 2004). The two-phase pressure drop keeps increasing as mass velocity and exit vapor quality rise.     

An experimental investigation of flow boiling in an asymmetrically heated rectangular microchannel

By using unique experimental techniques and carefully constructed experimental apparatus, the characteristics of flow boiling of water in microscale were investigated using a single horizontal rectangular microchannel. A polydimethylsiloxane rectangular microchannel (D_h = 103.5 and 133 μm) was fabricated by using the replica molding technique, a kind of soft lithography. A piecewise serpentine platinum microheater array on a Pyrex substrate was fabricated with the surface micromachining MEMS technique. Real time flow visualization of the phase change phenomena inside the microchannel was performed using a high speed CCD camera with microscope. The experimental local boiling heat transfer coefficients were studied, and single bubble inception, growth, and departure, as well as elongated bubble behavior were analyzed to elucidate the microscale heat transfer mechanisms. Tests were performed for mass fluxes of 77.5, 154.9, and 309.8 kg/m² s and heat fluxes of 180–500 kW/m². The effects of mass flux, heat flux, and vapor qualities on flow boiling heat transfer in a microchannel were studied.     

Elongated bubbles in microchannels. Part I: Experimental study and modeling of elongated bubble velocity

The velocity of elongated vapor bubbles exiting two horizontal micro-evaporator channels with refrigerant R-134a was studied. Experiments with tube diameters of 509 and 790 μm, mass velocities from 200 to 1500 kg/m² s, vapor qualities from 2% to 19% and a nominal saturation temperature of 30 °C were analyzed with a fast, high-definition digital video camera. It was found from image processing of numerous videos that the elongated bubble velocity relative to that of homogeneous flow increased with increasing bubble length until a plateau was reached, and also increased with increasing channel diameter and increasing mass velocity. Furthermore an analytical model developed for a diabatic two-phase flow, has been proposed that is able to predict these trends. In addition, the model shows that the relative elongated bubble velocity should decrease with increasing pressure, which is consistent with the physics of two-phase flow.     

Theoretical analysis of pressure-drop type instabilities in an upflow boiling system with an exit restriction

 In this work, two-phase flow pressure-drop type instabilities in an upflow boiling system are studied theoretically. Dynamic simulations of the pressure-drop type instabilities require the knowledge of the steady state characteristics of the system in terms of the pressure drop versus the mass flow rate. In a boiling system with an exit restriction at the outlet of the boiling channel, the pressure drop through the system concentrates at the exit restriction. Therefore, the correlation of the pressure drop of the two-phase mixture flowing through the exit restriction (i.e. a sharp-edged orifice) is essential in the calculation of the total pressure drop of the system. A model for the exit restriction is developed and compared with the experimental results covering a wide range of vapor quality with different heat inputs and inlet subcoolings. The drift-flux model is adopted to predict the steady state characteristics of the boiling system. The dynamic oscillations of the quasi-static pressure-drop type instabilities in the boiling system are simulated and good predictions of the system stability boundary and oscillatory characteristics are obtained when compared with the experimental results. Received on 4 May 2000     

Hydrodynamic Characterization of Nickel Metal Foam,Part 1: Single-Phase Permeability

This article presents results of the investigation of the fluid dynamic behavior in CVD processed nickel metal foams. An experimental facility was developed to measure the single-phase permeability in nickel metal foams in Darcian flow regime. Data on permeability values of seven different nickel foam samples was obtained. The pore sizes of the foam were obtained with scanning electron microscope. By defining friction factor and Reynolds number based on the permeability length scale a correlation was obtained for the foam permeability in Darcian flow regime. The result from this study was compared with the correlations reported by other researchers, and was found to be in good agreement.