Dryout Characteristics Research Articles

Effect of localized dry-out on startup and steady-state operation characteristics of ethanol-based oscillating heat pipes

It’s a significant challenge to choose a suitable heat dissipation method in different electronics assembly environments requiring different heat transfer distances. Oscillating heat pipes are promising heat exchangers, but localized dry-out during operation leads to an unstable heat transfer and limits further application of oscillating heat pipes. To extend the applicability of oscillating heat pipes, it is essential to reveal the localized dry-out characteristics under various heat transfer distances. In this study, the impact of localized dry-out on the multi-stage operating characteristics of ethanol-oscillating heat pipes with various heat transfer distances was investigated through partial visualization experiments. The anti-dry-out capacity of ethanol-oscillating heat pipes was quantitatively evaluated and compared with that of water-oscillating heat pipes. The experimental results revealed that the dry-out performance was closely related to the flow motion of the operating fluid. The stagnation of the operating fluid was the primary cause of localized dry-out. Localized dry-out deteriorated the startup and heat transfer performances of oscillating heat pipes. The anti-dry-out capacity first improved and then diminished as the heat transfer distance increased. The results also revealed that ethanol exhibited superior dry-out performance compared to water, especially at long heat transfer distances. Besides, an empirical correlation equation was fitted to evaluate the anti-dry-out capacity of oscillating heat pipes under different heat transfer distances based on the experimental results. The empirical correlation equation about the Ku number could be a reasonable predictor of the anti-dry-out capacity of oscillating heat pipes. The predicted results were in good agreement with the experimental results, with 86 % of the data deviating within ±20 % and an average standard deviation of 19.72 %. This research closed the gap in the prediction for the anti-dry-out capacity and provided a valuable reference to broaden the applicability of oscillating heat pipes.

Experiment study on dryout characteristics and thermal resistance analysis of two-phase closed thermosiphon

This study aims to explore the dryout characteristics of two-phase closed thermosiphon. It involves the design and experimentation on low-liquid filled heat pipes. We investigated the effects of different fill ratios, heating water temperatures, and cooling water flow rates on the start-up characteristics of two-phase closed thermosiphon. Based on the network model, we proposed an analytical expression for the thermal resistance, R, of two-phase closed thermosiphon, and provided an explanation of the various thermal resistance components. This research expands our understanding of dryout and provides insights for optimizing heat pipe design and addressing heat conduction issues with low fill ratios.

Dry-out characteristics of debris bed under different water injection conditions

A porous debris bed may form through the interaction between melting corium and coolant during a pressurized water reactor severe accident, which continues to emit large amounts of decay heat. The cooling of debris bed is a critical phenomenon and serves as a potential mitigation measure to halt the accident. In this paper, an experimental facility DBC was constructed to study the cooling properties and dry-out heat flux under different water injection conditions and system pressures. The debris bed consisted of stone particles ranging in diameter from 1～10 mm, with a porosity of 0.37. In the debris bed, a total of thirty-three electrical heating elements, twelve optical fibers equipped with 204 temperature sensors, and thirty-three thermocouples were embedded. Additionally, a computer program MIDEC was developed to investigate the vapor-liquid flow characteristics and dry-out mechanism. The experimental results indicated that the cooling of debris bed can be significantly improved by increasing system pressure. Furthermore, it was observed that the dry-out heat flux under bottom water injection conditions was considerably higher compared to that of top water reflooding. Under top reflooding conditions, the counter-current flow of vapor-liquid impeded the downward flow of coolant, resulting in insufficient coolant supply and leading to a dry-out phenomenon at the bottom region. However, under bottom injection conditions, both vapor and liquid flowed in an upward direction. As a result, the void fraction reached its maximum value at the top region of the debris bed. The distinct vapor-liquid flow characteristics under top and bottom injection conditions resulted in notable discrepancies in dry-out heat flux and dry-out positions. Based on the comparison between the calculated dry-out heat flux (DHF) and experimental results, it was observed that the Reed model accurately predicted the cooling of debris bed under top-flooding conditions when compared with measured data obtained from DBC experiments. However, for very small particle sizes, the Lipinski model would be more suitable and provided better predictions.

Flow boiling heat transfer and dryout characteristics of ammonia in a horizontal smooth mini-tube

A boiling two-phase flow heat transfer experimental system for ammonia in a horizontal smooth tube with an inner diameter of 3 mm was established. The purpose is to explore the characteristics of flow boiling and dryout phenomenon. The experimental conditions: heat flux density is 10–30 kW·m−2, the mass flux is 40–200 kg·m−2·s−1, the saturation temperature is −10–10 °C, and the range of vapor quality is 0.1–1. The results show that the dominant heat transfer in different flow patterns is different, intermittent flow with nucleate boiling, annular flow and mist flow with convective boiling; the increase of mass flux can reinforce the convective boiling, the heat flux density can enhance the nucleate boiling, the increase of saturation temperature has no significant effect on the nucleate boiling, but it can weaken the convective boiling. The increase of mass flux and heat flux density will increase the friction pressure drop, and the increase of saturation temperature can decrease the friction pressure drop. The dryout visualization results and attenuation of heat transfer show that the increase of saturation temperature, heat flux density and mass flux can decrease the initial vapor quality of dryout, which is consistent with the results of Mori et al. model. Compared with the experimental data, the existing two-phase flow correlations are not suitable for the post-dryout prediction for ammonia, and the Wojtan et al. model and Mori et al. model can provide a reference for the dryout prediction.

Dryout characteristics of low-GWP working fluids at low mass and heat fluxes in a vertical 4 mm diameter tube

The dryout phenomena in flow boiling of low-GWP working fluids at low mass and heat fluxes in a vertical 4 mm diameter tube were examined, and a new correlation of the dryout incipience quality has been proposed. The working fluids examined in the present study included R1233zd(E), R1224yd, R245fa, and HFE347pc-f, under the mass flux, the heat flux, and the reduced pressure ranging in 20–40 kg/(m2⋅s), 6–20 kW/m2, and 0.061–0.138 respectively. The dryout incipience quality was quantified based on the measured local heat transfer coefficients, and the flow boiling behavior near the dryout incipience was visualized through a transparent quartz glass tube with ALD-AZO electrodes on its inner wall. It is found that the dryout incipience quality decreases with increasing the heat flux, decreasing the mass flux, and increasing the reduced pressure. Under the same operating condition, R1233zd(E) shows the smallest value of the dryout incipience quality. From the flow boiling visualization, the dependence of the operating condition and the working fluids upon the dryout incipience is found to be associated with features of the entrained droplets in the vapor core. A new correlation of the dryout incipience quality based on the boiling number, the Weber number, the Reynolds number and the density ratio has been proposed. The new model predicts all the experimental data to within ±10%.

Boiling flow patterns and dry-out characteristics of R-1234ze(E) in a plate heat exchanger

ABSTRACT In this study, boiling flow patterns and dry-out characteristics of R-1234ze(E) in a plate heat exchanger (PHE) are investigated through visualizations and surface temperature measurements. The boiling flow patterns in the PHE are classified into three regimes by reflecting an evident flow direction and intermittent flooding (IF): rough liquid film flow, pulsating annular flow, and steady annular flow. Boiling mechanisms in the liquid-preferred and vapor-preferred paths are analyzed in a single channel. A boiling flow pattern map of R-1234ze(E) in the PHE is developed in terms of liquid and vapor superficial momentums, and the criteria for the transitions of the flow regimes are proposed to provide a reference for predicting flow characteristics during phase changes. Based on the measured surface temperatures, the partial dry-out is observed at medium to high vapor qualities in the center of the vapor-preferred path owing to the lack of the liquid phase. In addition, the relationships between the IF, partial dry-out, and heat transfer coefficients are comprehensively analyzed with physical interpretations.

On-route to dryout through sequential interfacial dynamics in annular flow boiling around temperature and heat flux controlled heater rod

A numerical methodology is used to track the interfacial dynamics of the dryout mechanism for flow boiling around the heated cylindrical rod. The mathematical model is based on the Volume of Fluid interface capturing technique for two-phases of fluids. A single set of Navier-Stokes equations with an added source term to take care of vapor mass flux generation due to phase change is utilized. The simulations are carried out for various combinations of flow parameters and heating conditions at a representative higher working pressure of 40 bar, which is usually observed in Boiling Water Reactors (BWR). Various mechanisms of dryout, such as disturbance wave generation, droplet entrainment, deposition, rewetting, nucleation beneath the thin annular film, and its dynamics are illustrated with pictorial representations. The liquid phase fraction distribution across radial and axial planes, wall temperature, and heat transfer coefficient are used to quantify the dryout characteristics.

Effects of the surfactant solution on the performance of the pulsating heat pipe

The startup, operation and dry-out characteristics of pulsating heat pipes (PHPs) are influenced by many parameters, and researches on improving its performance by optimizing the parameters, especially the thermal-physical properties of working fluids, have drawn wide attention. In this paper, the effects of deionized water (DI water) and the surfactant solution on the startup, operation and dry-out characteristics of the PHP were experimentally investigated at the charge ratios of (30.9–67.9)% and the heat fluxes of (637–21,083) W/m2. Sodium stearate was used as the surfactant, and three different concentrations of the surfactant solutions (10 ppm, 20 ppm and 40 ppm) were prepared. The experimental results indicated that the PHP with the surfactant solutions could start at a lower heat flux, compared with the DI water-PHP. When the concentration of the solution was 20 ppm, the PHP had the lowest thermal resistance within the tested conditions. Compared with that of the DI water-PHP, the thermal resistance of the PHP with 20 ppm surfactant solution decreased by 27.8% at the charge ratio of 56.7% and the heat flux of 3503 W/m2. The results also suggested that the PHP with the surfactant solutions presented higher dry-out heat fluxes, and showed relatively stable temperature oscillation characteristics. At the charge ratio of 67.9%, the dry-out heat flux of the PHP with the surfactant solutions increased by 17.80% compared with that of the DI water-PHP.

Flow boiling of R452A: Heat transfer data, dry-out characteristics and a correlation

This paper presents an experimental investigation on two-phase heat transfer and dry-out occurrence for refrigerant R452A in a single horizontal circular stainless-steel tube having an internal diameter of 6.0 mm. The effects of mass flux (from 150 to 600 kg/m2s), saturation (bubble) temperature (from 23 to 55 °C) and heat flux (from 10 to 65 kW/m2) are investigated and discussed. Heat transfer coefficient and dry-out vapor quality data are then compared to R404A results in the same operating conditions, observing that the nucleate boiling contribution of the new blend is penalized by its very high temperature glide during evaporation. The assessment of some dry-out and flow boiling heat transfer coefficient prediction methods is finally carried-out and a correction factor on the nucleate boiling term is proposed to take into account the negative effect of the temperature glide difference on the mass diffusion in the liquid. By implementing this modification on two chosen asymptotic models, the statistical error analysis is considerably improved.

Experimental study on dryout characteristics of steam-water flow in vertical helical coils with small coil diameters

Abstract Helically coiled heat exchangers have been widely used in industrial applications. The goal of this study was to understand dryout characteristics of steam-water flow in vertical helical coils with small coil diameters. Experimental investigations were performed in helical coils of 12.5 and 14.5 mm inside diameters. The coil diameters were 180, 280 and 380 mm. Experimental results were presented for the effects of flow parameters and geometric parameters, tube diameter and coil diameter, on dryout characteristics in helically coiled tubes. The results indicated that a partial dryout region exists because of the geometrical characteristics of the helically coiled tube, and secondary flow has great influence on critical dryout quality. For flow parameters, increase in mass flux increases dryout quality and increase in heat flux or system pressure decreases dryout quality. Dryout characteristics are also significantly influenced by the geometric parameters. The dryout quality increases with increasing hydraulic diameter and decreasing coil diameter. None of the available dryout correlations in the literature are able to predict the dryout quality well in helical coils with small coil diameters under the investigated experimental conditions.

A research on the dryout characteristics of CO2'S flow boiling heat transfer process in mini-channels

The experimental and theoretical researches have been carried out to get the flow boiling heat transfer characteristics of carbon dioxide (CO2 or R744) as a refrigerant in horizontal mini-channel. Based on infrared thermal imaging tests and experimental studies on heat transfer coefficients, the heat transfer coefficients and dryout characteristics of CO2 are analyzed qualitatively and quantitatively in following conditions: Heat flux: 2~35 kW m-2, Mass flux: 50~1350 kg m-2 s-1, saturation temperature: −10~15 °C, mini-channel inner diameter: 1 mm and 2 mm. Primary conclusions can be drawn from the results of the experiments: The increase of heat flux enhances the nucleate boiling heat transfer of the refrigerant inside mini-channel, which leads to the remarkable increase of heat transfer coefficient. But it speeds up the process of dryout. It also has a certain influence on vapor qualities of dryout at both the starting and the ending stage. The effect of mass flux on heat transfer enhancement depends on the dominant heat transfer mode in the tube. With the increase of mass flow rate, the vapor quality at the start of dryout has a decreasing trend. But the heat transfer coefficient increases at the end of dryout process or even after dryout process; the heat transfer coefficient does not vary monotonically with the saturation temperature: when the saturation temperature is high and even close to CO2's critical temperature, the heat transfer coefficient increases with the increase of saturation temperature; when the saturation temperature is low, the heat transfer coefficient increases with the decrease of saturation temperature. Besides, during the heat transfer process, the dryout vapor quality falls monotonically with the increase of saturation temperature. It is reasonable to conclude that dryout characteristics have significant influence on heat transfer coefficient. Fang correlation that predicts the heat transfer coefficient of CO2 is in good agreement with the experimental data, which has a mean absolute deviation of 15.7%, and predicts 71.98% of the entire database within ±20% and 86.84% of the entire database within ±30%.

Dryout-type critical heat flux in vertical upward annular flow: effects of entrainment rate, initial entrained fraction and diameter

This study aims to reveal the effects of liquid entrainment, initial entrained fraction and tube diameter on liquid film dryout in vertical upward annular flow for flow boiling. Entrainment and deposition rates of droplets were included in mass conservation equations to estimate the local liquid film mass flux in annular flow, and the critical vapor quality at dryout conditions. Different entrainment rate correlations were evaluated using flow boiling data of water and organic liquids including n-pentane, iso-octane and R134a. Effect of the initial entrained fraction (IEF) at the churn-to-annular flow transition was also investigated. A transitional Boiling number was proposed to separate the IEF-sensitive region at high Boiling numbers and the IEF-insensitive region at low Boiling numbers. Besides, the diameter effect on dryout vapor quality was studied. The dryout vapor quality increases with decreasing tube diameter. It needs to be pointed out that the dryout characteristics of submillimeter channels might be different because of different mechanisms of dryout, i.e., drying of liquid film underneath long vapor slugs and flow boiling instabilities.

Dryout characteristics of natural and synthetic refrigerants in single vertical mini-channels

Experimental results on dryout of seven refrigerants (R134a, R1234yf, R152a, R22, R245fa, R290 and R600a) in small, single vertical tubes under upward flow conditions are reported in this study. The experiments were conducted under a wide range of operating conditions in stainless steel tubes (0.64–1.70mm and 213–245mm heated length). The effects of operating parameters like mass flux, vapor quality, saturation pressure and channel size are discussed in detail. In general, dryout heat flux increased with increasing mass flux, and with increasing tube diameter. No effect of varying saturation temperature was observed. The experimental findings were compared with well-known macro and micro-scale correlations from the literature and it was found that Wu’s correlation (in modified form) quite satisfactorily predicted the whole database. A new correlation for prediction of heat flux at dryout conditions is also proposed.

Flow Boiling Heat Transfer and Dryout Characteristics of R600a in a Vertical Minichannel

Refrigerant-related environmental concerns forced legislative bodies to phase out some types of refrigerants, namely, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) and in the near future European legislation will be affecting hydrofluorocarbons (HFCs) as well. Natural refrigerants such as hydrocarbons can thus be expected to be more common as refrigerants in the future. Experimental findings on flow boiling heat transfer and dryout characteristics of isobutane (R600a) in a uniformly heated, vertical, stainless-steel test section (1.60 mm inside diameter and 245 mm heated length) are reported in this article. The experiments were conducted at two saturation pressures corresponding to the temperatures of 27 and 32°C, with five mass fluxes in the range 50–350 kg/m2-s and at outlet vapor qualities up to dryout conditions. Analysis showed that heat transfer was primarily controlled by the applied heat flux with insignificant effect of mass flux and vapor quality. The dryout heat flux increased with increasing mass flux; however, no significant effect of varying saturation temperature was observed. The experimental results (for heat transfer and dryout) were compared with different macro and microscale correlations from the literature.

Flow boiling heat transfer, pressure drop and dryout characteristics of R1234yf: Experimental results and predictions

Flow boiling heat transfer, pressure drop and dryout characteristics of R1234yf in a vertical stainless steel test section (1.60mm inside diameter and 245mm heated length) under upward flow conditions are reported in this article. The experiments were carried out at 27 and 32°C saturation temperatures with five mass fluxes in the range of 100–500kg/m2s while the applied heat flux was in the range of 5–130kW/m2. The experiments were carried out with gradual increase of the applied heat flux til completion of dryout. Under similar conditions, tests were repeated with R134a in the same test setup to compare thermal performance of these two refrigerants. The results showed that boiling heat transfer was strongly controlled by the applied heat flux and operating pressure with insignificant dependence on mass flux and vapor quality. The frictional pressure drop increased with mass flux and vapor quality and decreased with increasing saturation temperature as expected. Signs of dryout first appeared at vapor qualities of 85%, with the values generally increasing with increasing mass flux. The effect of varying system pressure was insignificant. The experimental results (boiling heat transfer, pressure drop and dryout heat flux) were compared with the predictions from well-known correlations (for macro and micro-scale channels) from the literature.

Dryout of counter-current two-phase flow in a vertical tube

Dryout of counter-current two-phase flow is one of key issues in chemical and/or industrial systems. The aim of the present study is to provide insights into the dryout phenomena of such counter-current two-phase flow of binary gas and liquid in a vertical tube. Experimental results suggest a significant influence of the subcooling of water and the binary gas flow rate on the dryout characteristics. The observed dryout was classified into three modes, i.e., film breakdown, dryout due to flooding and dryout due to critical flooding. At the low inlet air flow rate, dryout characteristics were affected by subcooling of water. The test section was partially overheated owing to the liquid film breakdown due to the Marangoni effect. At the moderate inlet air flow rate, the dryout is induced by the reduction on the falling liquid film due to flooding. At the high gas flow rate, most of the water was carried over due to the critical flooding. In the latter two cases, there was no influence of subcooling of water on dryout. These dryout phenomena were successfully realized in the numerical simulation based on the one-dimensional heat and mass transfer model, i.e., energy balance of a heat input, a heat release rate from the heating surface and a mass transfer rate due to phase change.

Numerical Evaluation of Two-Phase Heat and Fluid Flow of Widely Used Refrigerants

The pressure drop, heat transfer and dryout characteristics of five different refrigerants in a 1-cm-diam tube are numerically investigated using a novel approach to model the mass transfer associated with ebullition in the liquid film using computational fluid dynamics. The four different mass transfer mechanisms, namely, entrainment, deposition, evaporation, and condensation, are included in the computational model. The model is validated through comparison with experimental results of Stevanovic et al. as well as those of Lee and Lee. A comparison of the numerical predictions for the four R refrigerants with carbon dioxide reveals that the carbon dioxide consistently has longer dryout lengths and higher heat transfer rates for a given heat flux and operating pressure.

Experimental study of flow boiling heat transfer and dryout characteristics at low mass flux in helically-coiled tubes

The goals of this study were to understand flow boiling phenomena in helically-coiled tubes and to predict their flow boiling heat transfer coefficients (HTC) and dryout qualities. Flow heat transfer and dryout were experimentally investigated in coiled tubes of 12-mm inside diameter and 17-mm outside diameter. The helical diameters Dh of the coiled tubes used were 606mm and 977mm for the HTC experiments and 606mm, 977mm, and 1290mm for the dryout experiments. In flow boiling experiments mass flux G was 88.4–530.5kg/(m2s), system pressure P was 1–6MPa, and heat flux q″ to the tube wall was 30.0–1145.3kW/m2. For dryout experiments, q″ was 131.3–1764.9kW/m2. Flow boiling heat transfer in the coiled tubes was significantly affected by nucleate boiling and convective boiling, and not by secondary flow due to the helical coiling. HTC in the coiled tubes were accurately predicted (standard deviation 29%) by Steiner and Taborek HTC correlation for straight vertical tubes. Dryout qualities were significantly influenced by the coiled tubes. The effects of Dh, G, and P were greatly affected by secondary flow caused by the coiled tubes. Dryout in the coiled tubes can be only predicted using dryout correlations for them. The dryout map proposed by Berthoud and Jayanti was modified to consider the effects of redeposition and gravity; this modified correlation accurately predicted dryout qualities (standard deviation 10.1%) within the range of experimental conditions. These results will help guide design of cooling systems that use helical coils.

Flow boiling heat transfer and dryout characteristics of R152a in a vertical mini-channel

This article reports on flow boiling heat transfer and dryout characteristics of R152a in a vertical mini-channel. The experiments were carried out with a resistively heated stainless steel tube (1.60mm in diameter and 245mm heated length) at 27 and 32°C saturation temperature. Five mass fluxes in the range 100–500kg/m2s with heat fluxes from 5 to 245kW/m2 were tested. Under similar operating conditions experiments were repeated with R134a in the same setup to compare thermal performance of R152a. The results showed that the heat transfer was strongly influenced by the applied heat flux with insignificant convective contributions. The dryout heat flux increased with increasing mass flux but no effect of varying operating pressure was noticed. The experimental results for heat transfer and dryout heat flux were compared with well-known macro and micro-scale correlations from the literature.

Dryout characteristics during flow boiling of R134a in vertical circular minichannels

In this paper, the experimental results of dryout during flow boiling in minichannels are reported and analysed. Experiments were carried out in vertical circular minichannels with internal diameters of 1.22 mm and 1.70 mm and a fixed heated length of 220 mm. R134a was used as working fluid. Mass flux was varied from 50 kg/m 2 s to 600 kg/m 2 s and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 °C and 32 °C. Experimental results show that the dryout heat flux increases with mass flux and decreases with tube diameter while system pressure has no clear effect for the range of experimental conditions covered. Finally, the prediction capabilities of the well known critical heat flux (CHF) correlations are also tested.