Exit Vapor Quality Research Articles

Flow boiling pressure drop correlation in small to micro passages

An accurate and acceptable correlation for the prediction of two-phase pressure drop is considered a crucial step in heat exchanger design. Although many existing models and correlations were developed and proposed in the past, their ability to predict within acceptable error bands when applied in general to flow boiling flows is limited, even within their originally recorded ranges. The discrepancies are worse when predicting two-phase pressure drop in small to micro-scale passages. Therefore, the aim of the work described in this paper is to assess the most well-known models and correlations using a large experimental data-bank. The data-bank includes four refrigerants, namely R134a, R245fa, HFE-7100 and HFE-7200, small to micro heat exchangers made of different metals and different channel configurations. In addition, the data cover a large range of operating conditions, which can allow generally applicability of new correlations developed. This range covers channel hydraulic diameter of 0.46−4.26 mm, heated length of 20−500 mm, system pressure of 1 − 14 bar, mass flux of 50−700 kg/m2 s, wall heat flux ranging from 2 to 234 kW/m2 and exit vapour quality up to one. Twenty six existing models and correlations that were developed and proposed for vertical/horizontal flow, single/multi-channels and circular/non-circular channels were evaluated. Moreover, the effect of using different equations for calculating the two-phase mixture viscosity, void fraction, Fanning friction factor and Lockhart–Martinelli parameter was assessed and discussed. The mean absolute error of the existing correlations when compared with our data-bank was more than 30 %. Therefore, a new correlation for calculating the two-phase multiplier, which was strongly dependent on the Boiling number and the Lockhart–Martinelli parameter, was developed and then the frictional component and total two-phase pressure drop relationships were completed.

FLOW BOILING HEAT TRANSFER IN MICROCHANNEL HEAT EXCHANGERS WITH MICRO POROUS COATING SURFACE

This study experimentally investigated the heat transfer and pressure drop performance of refrigerant HFC-245fa flow boiling in microchannel heat exchangers with and without microporous coating. The flow boiling heat transfer performance at various mass fluxes, heating rate, exit vapor qualities, and surface coating thickness was compared. The test results showed that the 52 μm coating thickness porous surface exhibited 65-148% higher heat transfer performance than the smooth surface, while the 98 μm microporous coating surface showed a lower improvement of 41-90% at various mass fluxes. Partial dry out was observed at high and moderate mass fluxes on both smooth and porous coating channels. It happened at lower exit vapor qualities in microporous coating channels than that in smooth channels. The partial dry-out exit vapor qualities increased with decreasing mass fluxes. For the lowest mass flux, because of the low heat flux and low nucleation suppression, no significant partial dry out was investigated. The pressure drops in microporous coating channels were around 25-47% higher than those in smooth channels. There was no significant influence of microporous coating layer thickness on flow boiling pressures drops.

Experimental and numerical investigations of heat transfer and flow characteristics during flow boiling in straight and diverging PDMS microchannel

Recent trends of miniaturization and densely packed circuits have necessitated the development of novel cooling methods, especially for emerging fields like flexible microelectronics, biotechnology, and nanotechnology. A polydimethylsiloxane (PDMS) based microchannel flow boiling heat sink can be used for these applications due to its unique properties as such as unique mechanical properties, biocompatibility, optical transparency, etc. PDMS microchannels are fabricated using soft lithography techniques, which involve using a mould to pattern the PDMS material. The mould is created using photolithography, and the PDMS is poured over the mould and cured to create the microchannels. In this study, an effort is made to compare the flow and heat transfer performance of a PDMS microchannel heat sink with different channel shapes, i.e. straight and diverging microchannel heat sinks for different heat flux and mass flux conditions. The PDMS microchannel was fabricated using the soft lithography technique and a flexible laser-induced graphene heater was fabricated and integrated with the developed PDMS microchannel to act as a heat source. Experimentally, the parameters like bubble pattern, pressure drop, exit vapour quality, and corresponding heat transfer coefficient were determined at varying mass and heat flux ranging from 19 to 114 kg m−2s−1 and from 13.3 to 156.6 kW m−2 respectively for both the microchannels. It was observed that the flow in the diverging microchannel is more stable and has a 13% higher heat transfer coefficient value compared to the straight microchannel under similar conditions. Further, a 3D numerical study was carried out to corroborate and elaborate on the results. The heat transfer, flow characteristics, and bubble pattern results show an excellent agreement with experimental results. This work has significant relevance in designing flexible microchannel heat sinks.

Experimental Study on Flow Boiling Characteristics in Continuous and Segmented Microchannels with Vapor Venting Membrane

Flow boiling in microchannels is one of the promising techniques for heat dissipation occurred in micro devices. However, the rapid bubble growth must be suppressed, which leads to serious boiling instabilities, high pressure drop, and low heat transfer coefficient. The addition of porous hydrophobic membrane has proven an effective method to remove the vapor in-site in the literature. However, the effects of heat sink’s topological structures on the vapor venting are still a research gap. The present study experimentally investigates the influence of Polytetrafluoroethylene (PTFE) membrane on fluid flow pattern, pressure drop, vapor venting performance, and heat transfer characteristics of flow boiling in the continuous and segmented heat sinks. Results show that the vapor venting membrane can reduce the pressure drop and increase the heat transfer coefficient effectively by decreasing the exit vapor quality, especially in combination with the segmented structures. The interconnection area as a space for bubble growth and coalescence is beneficial for vapor venting due to increased vapor pressure and quantity. Following the enhanced vapor discharge, the fluctuation of pressure drop is further weakened, which is conducive for the safe operation of heat sink.

Annular liquid film thermohydraulic and intermittent dryout in microchannel flow boiling

The annular flow regime in microchannels is commonly described as the steady flow of a vapor core with a surrounding liquid film along the channel length. However, limited studies have suggested temporal discontinuity in the liquid film coverage of the microchannel wall. The lack of experimental tools to directly interrogate the liquid flow characteristics and the local surface heat flux has resulted in proliferation of different hypotheses concerning the physics of this process. Surface de-wetting due to high shear stresses at the vapor–liquid interface and liquid film instability and rupture due to perturbation growth at the vapor–liquid interface have been suggested as the underlaying causes of this phenomenon. In this study, a recently developed microsensor array capable of measuring the surface local heat flux and liquid film thickness and velocity is utilized to decipher thermohydraulic characteristics of the annular flow regime and intermittent dryout with unprecedented details. The studies are conducted in a 600 μm square cross-section microchannel using FC-72 as the test fluid at 70–80% exit vapor quality. The results show that liquid film thinning because of evaporation rather than de-wetting and film rupture is responsible for intermittent surface dryout. Studies at different mass and heat fluxes suggest that for a given surface heat flux, there is a mass flux threshold above which intermittent surface dryout vanishes. Furthermore, the experimental liquid and vapor flow velocities and liquid film thickness are used to calculate the liquid–vapor interfacial shear stress. The results suggest that under the studied conditions, the liquid–vapor interfacial shear stress has little impact on the surface intermittent dryout. This study enhances predictable and reliable design and operation of two-phase heat sinks at high exit vapor qualities.

Silicon microchannels flow boiling enhanced via microporous decorated sidewalls

Silicon microchannels with microporous decorated sidewalls were applied to enhance water flow boiling heat transfer, especially the critical heat flux (CHF). In experiments, a CHF of 473 W/cm2 based on the heater areas (210 W/cm2 if based on the effective heating areas) was achieved in the proposed microchannels at the mass flux of 389 kg/cm2, accounting for a 205% CHF enhancement compared with that in plain-wall microchannels without compromising the pressure drop. In addition, an improvement of 228% in effective heat transfer coefficient was achieved in the early nucleate boiling regime at the mass flux of 113 kg/cm2 compared with the plain-wall microchannels owing to the increased nucleation site density on the porous walls. Moreover, a visualization study via a microscope-linked high-speed camera was conducted to investigate the mechanism of liquid distributions and wetting phenomena in porous wall microchannels. The sustainable liquid film induced by the microporous structure greatly delays the local dryout and promotes more efficient heat transfer mechanisms such as thin-film evaporation. The exit vapor quality of the proposed microchannels has been increased above 0.3 at the tested mass flux range 113–389 kg/m2s, two times higher than that in the plain-wall microchannels (∼0.1).

Experimental study on flow boiling characteristics of hybrid micro-channels with gradient distribution pillars and bypass

Aimed at efficient cooling of high power micro-electric devices, the flow boiling characteristics of a novel hybrid micro-channel with gradient distribution pin fins and bypass is experimentally investigated using HFE7000. The onset of flow boiling, flow pattern, pressure drop, heat transfer coefficient and temperature uniformity are elucidated with the heat flux from 3.6 to 45 W/cm2 and mass velocity from 567 to 1290 kg/m2•s. The results show that the upstream denser and smaller pin fins reduce pin-fin recirculation zone and put off the bubble nucleation to a higher wall superheat. At a given heat flux of 30 W/cm2, the pressure drop of the gradient pin fin-bypass is reduced by 43.2% compared to that in the uniform pin fin. A large exit vapor quality of the latter induces a longer taper two-phase flow length and a larger contribution of vapor acceleration loss. Notably, the flow boiling stability is improved at a higher heat flux by the gradient pin fin-bypass with inhibiting vapor backflow and promoting subcooled liquid supply. As a result, the addressed heat flux increases by 10 W/cm2. Whereas temperature uniformity is better for uniform pin fin due to a larger taper two-phase flow region.

Dimensional optimization of two-phase flow boiling in microchannel heat sinks

The heat transfer coefficient (HTC) of microchannel heat sinks (MHS) is higher than common heat sinks due to higher area to volume ratio. Its value for two-phase flow boiling is much superior to single-phase flow. In addition, the two-phase flow boiling provides uniform wall temperature close to the coolant’s saturation temperature in low vapor qualities. In the present study, a heat sink is optimized dimensionally after modeling of the boiling of R134a refrigerant in the microchannels. Firstly, mixture two-phase method along with the wall heat flux partitioning are utilized to introduce an applied thermal model to design MHSs. The heat sink mounted on the backside of an Intel core i7-900 desktop processor with dimensions of 19 mm×14.4 mm× 1 mm is numerically simulated to investigate the thermal performance. The HTC and the exit vapor quality are comparable with the available empirical correlations and first law of thermodynamics, respectively. Then the proposed model is developed to optimize the dimensions of the microchannels to design the heat sink with minimized wall temperature. Bound optimization by quadratic approximation (BOBYQA) method results in the optimized dimensions of the microchannels in the heat sink. Optimization of heat sink’s geometry in terms of the dimensions of the microchannels at various boundary conditions will be practical as the unique application of the model.

Experimental study of ammonia flow boiling in a vertical tube bundle: Part 3 – Enhanced dimple tube with 2/3rd height solid round PVC nonconductive rod

This paper presents the experimental results of saturated two-phase flow ammonia in a vertical shell and tube heat exchanger comprising of seven dimpled enhanced tubes with solid PVC nonconductive round rods inserted in the lower 2/3rd section of each tube creating 67% annular section. The source of heat was hot ethylene glycol/water solution flowing on the shell side. The range of saturation temperature for ammonia was -20 °C ≤ Tsat ≤ -2 °C, heat flux range 5 kW m−2 ≤ q˙ ≤ 45 kW m−2, exit vapor quality range 0.1 ≤ x ≤ 0.9 and mass flux 76.9 kg m−2 s−1≤ G ≤ 145.7 kg m−2 s−1. For each saturation temperature, the effects of heat flux and mass flux were studied while the exit vapor quality at each mass flux varied between 0.1 and 0.9. Around 40% increase in heat transfer performance was observed with saturation temperature between –20 and –2 °C at exit vapor quality of 0.7. Heat transfer performance also increased with heat flux at fixed mass flux at each saturation temperature while the performance showed initial increase with quality and then a drop at the onset of dry out.

Experimental study of ammonia flow boiling in a vertical tube bundle: Part 1 – Enhanced dimple tube

• Boiling of ammonia in a vertical shell and tube evaporator with enhanced dimpled tube bundle. • Study of wide range of experimental parameters. • Enhanced heat transfer coefficient compared to previous studies. This paper presents the experimental results of saturated two-phase flow ammonia in a vertical shell and tube heat exchanger comprising of seven dimpled enhanced tubes. The tests were conducted for a wide range of parameters. The heat load was provided by hot ethylene glycol/water solution flowing on the shell side. Saturation temperature of ammonia was -20°C ≤ T sat ≤ 2°C, heat flux range 5 kW m −2 ≤ q ˙ ≤ 45 kW m −2 , exit vapor quality range 0.1 ≤ x ≤ 0.9 and a mass flux range of 34.99 kg m −2 s −1 ≤ G ≤ 66.32 kg m −2 s −1 . For each saturation temperature, the effects of heat flux and mass flux were studied for the saturated temperature range while the exit vapor quality at each mass flux was varied between 0.2 and 0.9. Two phase heat transfer coefficient increased with saturation temperature and heat flux. For all experimental conditions, performance showed initial increase and then a classic drop at the onset of dry out. The dimple tube showed better heat transfer coefficient compared to the previous studies performed on plain tubes with ammonia.

Experimental study of ammonia flow boiling in a vertical tube bundle: Part 2 – Enhanced dimple tube with full length solid round PVC nonconductive rod

• Boiling of ammonia in a vertical dimpled tube bundle with round PVC nonconductive rods. • Study of wide range of experimental parameters. • Enhanced heat transfer coefficient compared to previous studies. This paper presents the experimental results of saturated two-phase flow ammonia in a vertical shell and tube heat exchanger comprising of seven dimpled enhanced tubes with solid PVC nonconductive round rods inserted in each tube creating an annular section. The source of heat was hot ethylene glycol/water solution flowing on the shell side. The range of saturation temperature for ammonia was –20 °C ≤ T sat ≤ –2 °C, heat flux range 5 kW m −2 ≤ q ˙ ≤ 45 kW m −2 , exit vapor quality range 0.1 ≤ x ≤ 0.8 and mass flux 76.9 kg m −2 s −1 ≤ G ≤ 145.7 kg m −2 s −1 . For each saturation temperature, the effects of heat flux and mass flux were studied while the exit vapor quality at each mass flux varied between 0.1 and 0.8. Two phase heat transfer coefficient increased with saturation temperature and heat flux. For all test runs, typical thermal performance was observed, i.e., initial increase and then a drop at the onset of dry out. Annular flow boiling showed better heat transfer coefficient compared to the dimple tube for quality x < 0.5.

A Comparison of Finite Element and Lumped Modeling Techniques to Analyze Flow Boiling in Microchannels

This study quantitatively compares the finite element (FE) and lumped modeling techniques to analyze phase-change and multiphase flow in a microchannel evaporator. We compare the one- and two-zone models in the lumped modeling approach by considering both pressure drop and phase change in the evaporator. The study investigates the deviation in results from the lumped model relative to the FE model for various evaporator heat loads. For the one-zone model, the relative errors in predicting the average density, pressure, temperature, and vapor quality increase with the evaporator heat load. This increase is mainly due to the nonlinear variation in the heat transfer coefficient and pressure drop, which are calculated with higher inaccuracies at larger evaporator heat loads. On the other hand, the liquid-phase region and the exit vapor quality are predicted more accurately. The overall errors using one- and two-zone lumped models were less than 30% when the exit qualities do not exceed 0.5. Since strategies involving flow boiling in evaporators typically avoid high exit vapor qualities to circumvent dry-out and critical heat flux, the use of lumped models under these conditions is favorable. This approach also avoids the computational costs associated with FE models while achieving sufficient accuracy to predict the evaporator’s performance.

Numerical modeling of an outdoor unit heat exchanger for residential heat pump systems with nonuniform airflow and refrigerant distribution

Multirow finned tube heat exchangers have been extensively used as an outdoor unit in heat pump applications. Several factors, including nonuniform airflow distribution, refrigerant circuitry arrangement, fin and tube configurations, and operating conditions, have a significant influence on its thermohydraulic performance. For accurate outdoor unit heat exchanger modeling and analysis, developing a robust and experimentally validated numerical model reflecting the influence of realistic nonuniform airflow and refrigerant distribution is necessary. This paper presents a general-purpose numerical model, implemented in MATLAB, for a multirow finned tube heat exchanger using a tube-by-tube modeling approach based on the log mean enthalpy difference method. State-of-the-art heat transfer and pressure drop correlations are used for each refrigerant-side's flow regime and airside fin configuration. An alternative iteration loop is also incorporated to solve the complex circuitry of the heat exchanger under wet conditions. Simulation results of the current model yield consistent and more real heat transfer, pressure drop, and temperature contours using nonuniform airflow velocity profiles than those of the simplified models that assumed uniform airflow. Therefore, higher accuracy of the performance evaluation for complex refrigerant circuitry was achieved. Furthermore, current model is able to identify the exit vapor quality that allows proper selection of tubes and mass flow rates to avoid unnecessary superheat or sub cooling. The model is also validated with available experimental data, and the maximum error is within ±10.0%.

Experimental study of natural circulation flow instability induced by flow boiling and loop circulation

An experimental study of natural circulation flow instability has been carried out to investigate instability modes at low pressure. Water in the primary loop is heated in a single macro-channel and condensed in a tube-shell shape condenser by circulating coolant in the secondary loop. Two kinds of flow instability modes are identified: flow instability induced by flow boiling in the heated channel and flow instability induced by loop circulation. Flow boiling instabilities are found at three exit vapor qualities: xe < 0 (sub-cooled boiling), 0 ≤ xe < 1 (saturated boiling) and xe = 1 (periodic dryout). Sub-cooled boiling instability is caused by bubble growth and condensation in the long sub-cooled flow boiling region. Saturated boiling instability is classified as Pressure Drop Oscillations because it operates in the negative slope region of internal characteristic curve. Combined with the oscillations of heating surface temperature, periodic dryout is confirmed by predicting CHF from empirical correlations. Flow instability induced by loop circulation is classified as Natural Circulation Oscillations, which is governed by the imbalance between heat transfer in the heated channel and condenser. The frequency and amplitude of flow boiling instability are given. The evolution of flow instability is indicated in the stability map.

Two-phase frictional pressure drop with pure refrigerants in vertical mini/micro-channels

Environmental concerns have urged a search for eco-friendly refrigerants in the refrigeration industry to overcome ozone depletion and global warming problems. Therefore, current research emphasizes frictional pressure drop during flow boiling of environment-friendly refrigerants (GWP<150), isobutane, HFC-152a, HFO-1234yf were tested against commonly reported HFC-134a. The data presented here was collected under heat flux-controlled conditions; the test piece was a round tube (1.60 mm diameter). The data collection was performed at 27 and 32 °C with mass velocities in 50–500 kg/m2s range. Effects of critical controlling parameters, like heat flux, mass velocity, exit vapor quality, operating pressure and medium, were studied in detail. It was observed that pressure drop increases along with mass velocity increment in the test piece and increases with exit vapor quality increment. The same was noticed to decrease with saturation temperature increment. Parametric effects and prediction of assessment methods are reported.

Two-Phase Heat Transfer and Flow Regimes in Pin Fin-Enhanced Microgaps—Effect of Pin Spacing

Abstract An experimental investigation of the flow boiling of dielectric refrigerant R245fa is conducted in microgaps with enhancement features. A silicon microgap of height 200 μm populated with pin fin arrays of diameter 150 μm with spacing 200 μm in both horizontal and vertical directions is examined. For five different test conditions and in a wide range of mass flux from 781 to 5210 kg/m2s, and inlet temperatures in the range of 13–18 °C, average single-phase and two-phase heat transfer coefficients, pressure drop, and exit vapor quality are reported. Three major flow patterns are observed in the pin finned area using high-speed flow visualization at frame rate of 2229 fps: foggy, bubbly, and slug flow. Based on the experimental data, a flow regime map is constructed.

Ultra-low pressure drop membrane-based heat sink with 1000 W/cm2 cooling capacity and 100% exit vapor quality

This paper presents the most recent progress on advancing the membrane-based heat sink (MHS) science. A model developed to predict the MHS performance suggested that the membrane permeability limited the critical heat flux (CHF). The new generation MHS discussed here benefits from a newly developed membrane with 7 times higher permeability compared to the membrane used in the previous generation MHS. Experimental studies were conducted on a heat sink with a heater surface area of 0.7 × 0.7 cm2. The results confirmed that a higher membrane permeability substantially increases the CHF of surface structures with enhanced wickability and surface area ratio (Ar) at low pressure drops. A maximum CHF of about 1000 W/cm2 was achieved on a surface with Ar = 3.45 at a supplied liquid pressure of only 4 kPa, 2.5 times higher than the CHF reached with the low permeability membrane used in the first generation MHS. The new membrane enhanced the heat transfer coefficient at low supplied liquid pressures but substantially less than the 1.8 MW/m2 K reached at the highest pressure (i.e. 20 kPa). This low pressure drop along with a heat sink exit vapor quality of 100% resulted in an extremely low pumping power. The ratio of CHF versus the theoretical pumping power is 1–2 orders of magnitude higher than that of the other two-phase heat sinks reported in the literature.

Thermal analysis and effects study of evaporator exit vapor quality in a separate heat pipe

A separate heat pipe is a kind of wickless heat pipe in which flow boiling occurs in the evaporator and flow condensation occurs in condenser. The evaporator and condenser can be installed in different places. Working fluid circulates in the heat pipe due to static pressure difference between the evaporator and condenser. The separate heat pipe is a high efficiency heat transfer device which can transport heat for a long distance without considerable losses. It is becoming more and more widely used in various fields, such as heat recovery, solar energy collection, air conditioning, passive cooling and so on. In the present study, a numerical model for a separate heat pipe is established and verified with experimental results. Reasonable agreement with MAE of 3.94% between the model and experimental data is obtained. Further studies depict that vapor quality at evaporator exit varies from 0.1 to 1 and shows great self-adjustment ability under different working conditions. Due to the self-adjustment, the separate heat pipe can maintain its heat transfer performance in the charge range of 0.55 m–1.35 m. And this range is strongly affected by both vapor quality at evaporator exit and liquid column height in the downcomer.

Nucleate flow boiling enhancement on engineered three-dimensional porous metallic structures in FC-72

Flow boiling heat transfer experiments were conducted in a closed-loop facility on engineered hollow structures using FC-72 as the working fluid. The use of Selective Laser Melting, an additive manufacturing technique, has allowed the fabrication of specialized 3D porous metallic structures with repeating unit cells of spherical voids and cylindrical connecting channels. Three porous substrates which consist of a small, large and gradient unit cells were fabricated. The porous substrates show superior flow boiling heat transfer as compared to a plain surface due to the increased number of nucleation sites and more vigorous flow mixing. For each substrate, the heat transfer coefficient shows an initial increasing trend from low to mid-range of the exit vapor quality and deteriorates thereafter when approaching the dry-out condition. Visualizations using a high-speed camera reveal that bubbly and churn flows were prevalent in the substrates and suggest the strong dominance of nucleate boiling over the convective mechanism. The Gradient-reverse substrate showed superior performance due to the channeling of the liquid-vapor mixture into smaller cross-sectional areas and the delay of dry-out for high vapor quality.

Boiling Heat Transfer Coefficients in a Falling Film Helical Coil Heat Exchanger for the NH3–LiNO3 Mixture

This paper reports the experimental data of boiling heat transfer coefficients for the ammonia–lithium nitrate mixture in a laminar falling film. The analyzed heat exchanger consists of a shell with an internal helical coil. More than one hundred test runs were carried out in steady-state conditions to determine the boiling heat transfer coefficients at generation temperatures, concentrations, and mass flow rates typical of absorption cooling systems of capacities between 5 and 10 kW. Ammonia vapor was produced at generation temperatures between 80 °C and 105 °C obtaining boiling heat transfer coefficients between 85 and 340 W/m2K. Semi-empirical correlations were used by diverse authors to correlate the experimental data. A new correlation was proposed with which the best adjustments were obtained. Also, the influence of the heat flux, the refrigerant solution mass flow rates, and the exit vapor qualities were analyzed in the boiling heat transfer coefficients.