Vapor Quality Research Articles

Fundamental research on the condensation heat transfer of the hydrocarbon-mixture energy in a spiral tube described by a universal model using flow pattern based and general modes

The transportation of liquefied natural gas (LNG) is a crucial aspect of global energy application and security. However, there are still some challenges in liquefaction. This study establishes systematic experiments to investigate the condensation two-phase flow and heat transfer process of hydrocarbon-mixture energy (methane/ethane/propane/isobutane) in a spiral tube. The main influencing parameters, including mass flux (200–560 kg/(m2·s)), operating pressure (2–4 MPa), and vapor quality (0–1) on the heat transfer intensity and frictional pressure drop were analyzed. Different two-phase flow patterns were observed and categorized them as bubble flow, intermittent flow, stratified-wavy flow, and annular flow. For bubble flow and intermittent flow, vapor quality is found to remarkably affect the heat transfer; regarding stratified-wavy flow and annular flow, gravitational force and inertia force are found to prominently influence the heat transfer, respectively. Additionally, a newly universal model includes split flow patterns mode (SFPM) and the general mode (GM) has been proposed, which is with high-precision for describing the condensation heat transfer coefficients of different flow patterns. Furthermore, the analytical results show the SFPM's root mean square error and mean absolute relative deviation are 97.35 W/(m2·K) and 0.036.

Experimental and Computational Investigation of Flow Boiling in a 52 μm Hydraulic Diameter Microchannel Evaporator With Inlet Restrictions and Heat Spreading

Abstract Microchannel flow boiling presents an effective thermal management strategy for high heat flux (>1 kW/cm2) devices. Fundamental mechanisms of microchannel flow boiling behaviors are difficult to determine due to macroscopic limitations of experimental hardware. In addition, flow stabilizing features of microchannel evaporators such as inlet restrictions and heat spreading further complicate fluid flow and heat transfer dynamics. Computational models, when utilized with experiments, can provide a more detailed understanding of behaviors which cannot be determined experimentally. The present study developed a computational model for flow boiling heat transfer in a 52 μm silicon microchannel evaporator designed to cool a laser diode bar, with inlet restrictions and a nonuniform heating profile at the channel level. A conjugate heat transfer model along with a coupled level set and volume of fluid (CLSVOF) model was created in ansysfluent and compared with experimental flow boiling data to gain further insights into the performance of a realistic microdevice. Heat spreading in the channel outside of the heater footprint was observed due to the high thermal conductivity of the silicon substrate. The inlet orifices impacted local flow patterns by creating a large pressure drop and forming a recirculation zone immediately downstream. This behavior resulted in pressure recovery zones and regions of separated flow boiling behavior. Bubbly, slug, and churn flows were seen to be dominant flow regimes. The heat transfer coefficient was found to be dependent on heat flux and flow regime, and more weakly on mass flux and outlet vapor quality.

Experimental investigation and prediction of CHF in wire-wrapped rod bundles

The wire-wrapped tight lattice fuel assembly is one of the options for small modular reactors. To clarify the characteristics of critical heat transfer for departure from nucleate boiling, experiments were conducted to investigate the critical heat flux in a wire-wrapped 19-rod bundle. The critical heat flux coincident with associated thermal–hydraulic parameters was obtained under typical pressurized water reactor conditions. Validation of data accuracy was performed to provide confidence in the experiment results. It was found that the critical heat flux in the wire-wrapped rod bundle is sensitive to the change in steam quality at some steam quality regions. Besides, the critical heat flux in wire-wrapped rod bundles has a similar reflection under the effects of local pressure, local mass flux, and steam quality. However, the current correlations tend to overestimate the critical heat flux for a wire-wrapped rod bundle. Hence, further development of KfK-3 correlations was proposed for better prediction of the critical heat flux in wire-wrapped bundles. This work is helpful for rod bundle designs of advanced pressurized water reactors.

Flow boiling heat transfer performance of R245fa in a vertical enhanced evaporator tube with sintering and electroplating composite coats

The R245fa flow boiling performance was investigated in a vertical evaporator tube of which inner wall surface is coated by a composite sintering and electroplating technology. The effects of vapor quality, heat flux, mass flux, and flow direction were compared and analyzed in the bare tube (BT) and the sintering and electroplating treated tube (SET). The results showed that the heat transfer coefficient increased and then decreased with the rise of vapor quality and heat flux. The heat transfer performance in downward flow was better than that in upward flow, while an opposite trend was observed for frictional pressure drop. With the increased of vapor quality, the frictional pressure drop increased and then decreased, but it monotonically increased with the increasing mass flux. The heat transfer enhancement mechanism was explored through flow pattern visualization. The annular flow pattern conversion line of SET was earlier than that of BT. Due to the strong wettability of SET, the thickness of liquid film was thinner in downward and the mother bubble was bigger in upward, which had a positive effect on flow boiling heat transfer. The maximum the heat transfer enhancement factor (EF) and the performance evaluation parameter (PEC) were 1.83 and 1.75, respectively.

Ammonia condensation in the horizontal and vertical smooth tubes

The paper proposes the experimental study of ammonia condensation in aluminium tubes with 8 mm and 11 mm internal diameters. The new study aimed to compare and extend the existing ammonia in-tube condensation data. The lines were tested mainly in horizontal configuration at mass velocities of 20 to 160 kg·m−2s−1, saturation temperature of 35 to 65 °C and inlet vapour quality of 0.1 to 0.9. Shah (1979), laminar-laminar Dobson and Chato (1998), and Cavallini et al. (2001) models predict well ammonia condensation HTCs at mass velocities of 120 to 160 kg·m−2s−1. A new phenomenological model has been proposed for the mass velocities of 20 to 120 kg·m−2s−1, predicting the test data with an accuracy of ±20 %. Similar ammonia condensation HTCs have been obtained for horizontal and upstream flows at a mass velocity above 120 kg·m−2s−1. The lowest condensation HTCs at the considered boundary conditions characterise the downstream configuration of an in-tube condenser at a mass velocity below 120 kg·m−2s−1. However, inclining the ID 8 mm tube by 15° from the normal substantially increases the condensation HTCs by 10 to 30 %, depending on vapour quality. At the inclinations of 15 to 45° from the horizontal line toward the downstream flow, the condensation HTCs exceed the ones of horizontal flow by 10 to 15 % at the vapour quality of <0.5 and low mass velocity of 80 kg·m−2s−1.

Experimental study of R600a boiling in microfin tube

This paper presents experiments to investigate flow boiling and pressure gradients of R600a. The study employed microfin and smooth copper tubes simulating conditions in a domestic refrigerator, using low mass fluxes (MFs) and saturation temperatures (Tsat). The test sections featured a tube length of (400 mm), an inner diameter of (4.3 mm), and an outer diameter of (5 mm). Test conditions varied, including MFs of (25, 35, and 45 kg m−2 s−1), vapor quality from (0.18–1), Tsat of (−15, −9, and −5 °C), and HFs of (10, 14, and 18 kW m−2). Both tubes shared identical dimensions, clearly comparing microfin impact on flow boiling characteristics. The study evaluated enhancement factor (EF), penalty factor (PF), and overall efficiency (η) by comparing heat transfer coefficients (HTC) and pressure gradients in the microfin tube versus the smooth tube. Results indicate a significantly boosted HTC in the microfin tube compared to the smooth tube, with a reasonable increase in pressure drop. Under specific conditions (HF of 10 kW m−2, Tsat of −9 °C, and MFs of 25 kg m−2 s−1), the average HTC is enhanced by 325%, while the pressure drop is increased by 150%. Experimental HTC and frictional pressure gradient comparisons aligned well with values predicted by existing literature correlations.

Flow Boiling Heat Transfer in Enhanced Tubes With Composite Structure

Abstract To fill the research gaps in the study of flow boiling on composite surfaces, four heat exchange tubes with different structures were investigated: an enhanced tube with herringbone teeth (EHT_HB), a tube with a dimple structure (EHT_DIM), a tube with a composite herringbone tooth/dimple structure (EHT_HB/DIM), and a smooth tube as a baseline for comparison purposes. The experimental conditions were set as follows: saturation temperature 6 °C, mass flux 50–205 kg/(m2 · s), and vapor quality 0.2–0.8. After confirming the validity of the experimental results, the effects of the mass flux on the flow boiling heat transfer and pressure drop characteristics were examined. Results showed that the flow boiling heat transfer coefficient and pressure drop increase with the increase of the mass flux and the vapor quality in the tested tubes. The EHT_HB/DIM tube was found to combine the advantages of the two structures and to have the highest heat transfer coefficient. The average value of the heat transfer coefficient was found to be 43.75% higher than that of the smooth tube. Also, the average flow boiling heat transfer coefficients of the EHT_HB and the EHT_DIM tubes were found to be 15% and 35% higher than that of the smooth tube, respectively. From the perspective of the frictional pressure drop, it was found that the EHT_HB/DIM tube exhibited the maximum frictional pressure drop. The optimum working conditions of the enhanced tubes were determined by introducing a performance factor that captures the combined effects of heat transfer and pressure drop.

Experimental investigation on the flow boiling of R134a in a plate heat exchanger with mini-wavy corrugations

As a common component in various applications, the two-phase heat transfer of plate heat exchanger has been frequently studied. However, further investigations are still required on the flow pattern, especially for those with new plate structures brought with the emergence of new applications. This study presented an experimental investigation on the flow boiling of R134a in a plate heat exchanger with mini-wavy corrugations designed for thermal management system of electric vehicle (EV). The flow pattern transitions in the test section were captured and discussed, meanwhile the effects of vapor quality and mass flux on the variation of heat transfer coefficient (HTC) were analyzed. Based on the observation, the two-phase flow pattern developed from pulsating film flow, then rough stream flow and finally to thin stream flow with the increase of vapor quality. The flow pattern map was determined with the superficial momentum of the liquid and vapor phase refrigerant, while the empirical correlations were developed based on two-phase Weber number to predict the transitions between the flow patterns. The HTC showed an intermediate increase firstly under low vapor qualities, and rapidly decreased after reaching the peak value at the vapor quality of 0.45–0.6. In addition, the experimental data were compared with the present correlations, and a new Nu correlation was developed considering the transitions of flow patterns, showing an optimal prediction on the experimental data with the mean absolute error of 11.3%.

Enhancing the vertical downward condensation heat transfer of a plate heat exchanger by electrochemical etching

This research employed electrochemical etching to generate microscale roughness on a plate heat exchanger (PHE) thereby enhancing condensation heat transfer in a vertical downward flow. The effects of the mass flux, vapor quality, heat flux, saturation temperature and microscale surface roughness on the condensation heat transfer coefficient (HTC) and frictional pressure drop (FPD) of R-513a flowing through a PHE was experimentally investigated. Experimental measurements were made under various heat fluxes of 5–15 kW/m2, mass fluxes of 60–80 kg/m2s, saturation temperatures of 40–50 °C, and mean vapor qualities of 0.1–0.7. HTC increased by up to 28, 26, 74, and 32.5 % as the heat flux, mass flux, average quality, and microscale surface roughness were respectively increased, but decreased by 19 % with the saturation temperature. FPD increased as the mass flux and average vapor quality increased, whereas it was reduced as the refrigerant saturation temperature increased. In contrast, the heat flux had little to no influence on FPD. Correlations for predicting the Nusselt number and friction factor were postulated by considering influential dimensionless numbers during heat transfer. Maximizing the Nusselt number and thermal performance factor was ultimately resolved using a multi-objective genetic algorithm to find a Pareto front solution. Consequently, the optimal surface roughness was found to be 9.94 μm.

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.

Numerical investigation of flow boiling behavior in a vertical tube of a shell‐and‐tube phase change heat storage unit

AbstractPhase Change Materials (PCMs) demonstrate significant potential as latent heat storage systems for direct steam generation. This investigation explores the heat transfer dynamics of an upward‐flowing heat transfer fluid (HTF) boiling within a vertical tube, concurrently examining the solidification process of PCM outside the tube. A two‐dimensional model of a shell and tube heat storage device is established for this purpose. The simulation employs the Volume of Fluid (VOF) model coupled with the Lee model to simulate HTF boiling inside the tube. Concurrently, the enthalpy‐porosity model is utilized to analyze the PCM solidification process. The findings reveal that PCM solidification and heat discharge are influenced by the temperature and thermal conductivity of the HTF, leading to uneven solidified PCM thickness along the tube's length over time. Additionally, variations in tube length, inlet velocity, and inlet temperature impact both the vapor quality of HTF and the thickness of solidified PCM. Tube length correlates with boiling time, affecting the accumulation of steam quality throughout the coupling process. Inlet velocity dictates the heat transfer of HTF with PCM within a limited distance, and insufficient heat exchange resulting in the cessation of HTF boiling. Inlet temperature determines the appropriate device size, lower inlet temperatures necessitate longer tubes to extend the heat charging duration until boiling initiation.

Ammonia single-phase and two-phase frictional pressure losses in horizontal smooth and straight inner-grooved tubes

The experimental study of single-phase and two-phase frictional pressure losses has been performed with ammonia flow in stainless steel smooth tubes (internal diameter of 5.5 mm and 8.5 mm) and six straight inner-grooved aluminium tubes with different nominal diameters (7.15 to 13.3 mm), length (0.45 to 2.0 mm) and thickness (0.35 to 0.8) of fins. Single-phase test boundary conditions correspond to Reynolds numbers of 1000 to 20,000. Two-phase tests have been carried out at saturation temperature of 45 to 65 °C, mass velocity of 50 to 210 kg·m−2s−1 and vapour quality of 0.1 to 0.9. Comparison of obtained test data with the existing frictional losses approaches shows that the Blasius model is appropriate to single-phase flows in smooth tubes, and Müller-Steinhagen and Heck's (1986) model is most accurate in predictions of two-phase frictional losses in smooth tubes. Existing approaches of single-phase frictional pressure loss predictions in the pipes and channels with longitudinal fins (Carnavos, 1980; Edwards and Jensen, 1994) are not adequately suited to the ammonia test points obtained in the considered straight inner-grooved tubes. A new semi-empirical model has been proposed, based on generalisation around the revealed geometric dimensionless factor, which accurately describes friction pressure losses in the considered inner-finned profiles. The simplest homogeneous model for defining two-phase friction pressure losses, using McAdams et al.'s (1942) recommendation for mixture dynamic viscosity definition and a new model of single-phase friction factor, best fit both quantitatively and qualitatively to ammonia two-phase test points in most of the considered straight inner-grooved tubes.

Flow boiling heat transfer characteristics and correlation development of R290/R600a mixtures in an internally threaded tube

Excessive hydrofluorocarbons (HFCs) usage significantly contributes to the global warming. Therefore, there is a widespread shift toward natural refrigerants with low global warming potential (GWP). Among these alternative refrigerants, R290, R600a, and their mixtures have gained considerable attention. However, their flammability limits the charge. To decrease the equipment volume and reduce the refrigerant charge, internally threaded tubes have been introduced to enhance the heat transfer. Therefore, in this study, flow boiling experiments were conducted on R290, R600a, and mixtures inside an internally threaded tube, obtaining heat transfer data under a pressure of 0.216–0.416 MPa, heat flux of 10.5–73.0 kW m−2, mass flux of 70–190 kg m−2 s−1. To quantitatively discern the dominant heat transfer mechanism, a dimensionless number Nz was introduced. In the nucleate boiling dominant region, an increase in the heat flux markedly enhances the heat transfer, with pressure and mass flux playing minor roles. Furthermore, the heat transfer deterioration trend of the mixtures aligns with the variation trends of concentration slip and boiling range. In the convective heat transfer dominant region, a rise in the mass flux significantly increases the heat transfer coefficient, while the pressure and heat flux have minor effects. Additionally, at high R290 mole fraction and vapor quality, high surface tension maintains a continuous liquid film, enhancing the heat transfer. Furthermore, new heat transfer correlations for flow boiling inside the internally threaded tubes were established, which exhibit a mean relative deviation of −2.30 % and 8.80 % for the obtained experimental results and collected data, respectively.

Gas-liquid two-phase flow pressure drop in flattened tubes: an experimental and numerical study

Experimental, numerical and empirical research is carried out on pressure drop features of air-water two-phase flow in horizontal flattened tubes. Circular tubes of 10.5 mm I.D. made of copper were successively flattened into inner heights of 9, 8, and 6 mm (AR=1.27, 1.5, and 2.2, respectively). The experiment operation conditions were 200, 500, and 1000 kg/m2s for mass velocity, 6, 8, 10 LPM for flow rate, and 0 to 0.005 for gas quality. Also, the pressure drop for R134a and R410A was estimated numerically using ANSYS Fluent. The simulation test condi-tions were for vapor quality of 0.1 to 0.9 and saturation temperature of 40°C, while the condi-tions for mass velocity and flowrate are taken as that of the experiment test. The experimental data were examined to see how different factors affect on the pressure gradient. According to the outcomes and as compared to the circular tube, the pressure gradient was raised up to 27%, 95%, and 218% for tubes flattened with aspect ratio of 1.27, 1.5, and 2.2, respectively. More-over, the pressure drop for either air-water or refrigerant fluids is increased dramatically with increasing flow rate, but it decreases with increasing vapor quality. When compared to known circular tube correlations, a good agreement was achieved. Finally, the minimum difference between the experimental, numerical, and correlated results was less than 3% for gas quality of 0.0048 and aspect ratio of 2.2.

Effect of steam quality on the oil recovery factor of the Karazhanbas field

Background: The injection of the thermal agent as "steam" reduces the viscosity of the oil and the mobility ratio, which increases the displacement and coverage factor, and ultimately significantly increases the oil recovery factor. This statement was the basis for determining the effect of steam enthalpy on the oil recovery factor, as well as for reliably determining the flow rate of flared gas, as a result of which the existing gas pipeline will be reconstructed, the required amount of fresh water will be calculated, and the distribution of steam pumping in the entire Karazhanbas field.&#x0D; Aim: The purpose of the article is to consider the influence of the quality of steam produced at the steam generator units of the Karazhanbas field on the oil recovery factor. The main parameter that determines the quality of steam is dryness, which was calculated using the actual values of the boiler (pressure, temperature, gas flow), flue gas tests on a special device, as well as laboratory tests to determine the heat value of the combusted gas.&#x0D; Materials and methods: This work used materials from GHM (geological and hydrodynamic modeling), results of studies of boiler flue gases and heat balance.&#x0D; Results: The calculation showed a low quality of the produced steam, which negatively affects the recovery factor.&#x0D; Conclusion: Evaluation to determine the optimal value of saturated steam parameters (steam dryness, temperature in terms of development efficiency) showed that the higher the temperature and dryness of steam at the wellhead and bottom hole of the injection well, the higher the oil recovery factor at the Karazhanbas field.

Maximizing hydrogen-rich syngas production from rubber wood biomass in an updraft fluidized bed gasifier: An advanced 3D simulation study

BackgroundThis research focuses on biomass gasification as a solution to the global energy crisis. The study aims to optimize the production of syngas by simulating an updraft fluidized bed gasifier using rubber wood as fuel. The goal is to maximize CO and H2 at the gasifier outlet by investigating different vertical positions and angles of the biomass feed inlet. MethodsThe research utilizes 3D simulation and Computational Fluid Dynamics (CFD) through the implementation of the ANSYS Fluent® software package. Realistic assumptions and the K-epsilon turbulence model simplify the analysis. The model assumes a steady state, adiabatic conditions, homogeneous gas phase kinetics, and ideal fluid behavior. Significant findingsThe optimal biomass feed inlet position is 200 mm, yielding higher CO, while a position of 250 mm produces more H2. A 45° feed inlet angle slightly improves syngas quantities but increases CO2. Using a gasifying agent with higher CO2 enhances combustion, while limited O2 facilitates gasification. Increasing oxygen content from 20 % to 50 % raises H2O content. More steam in the inlet boosts H2 production, but excessive H2O reduces steam quality, making expensive steam less effective than air. These findings offer valuable insights for optimizing fluidized bed biomass gasification systems.

Water Flow Boiling in Micro/Mini Channels Using Volume of Fluid Model

Recent advancements in computational fluid dynamics (CFD) have triggered research in the field of heat exchangers. Driven by the need to decrease the size of heat exchangers, many researchers have exploited the higher heat transfer achieved by replacing single-phase flow systems with boiling counterparts. The concept of using mini-channels to provide compact heat exchangers while maintaining heat transfer performance is relatively new. A minimal number of researchers have reported simulations of water-steam systems in mini-channels. This paper presents a numerical study of the heat transfer performance (HTP) of mini channels in a water-steam system using the volume of fluid (VOF) model coupled with the Lee phase change model on commercial CFD software ANSYS. The numerical model consisted of a 1 mm × 1.5 mm × 52 mm channel with boundary conditions: top adiabatic; constant heat flux at the bottom surface; left/right periodic; mass flow inlet and pressure outlet. A mesh independence study was carried out for the proposed model, and simulations were validated against the experimental results of heat transfer versus vapor quality for a wide range of mass and heat fluxes. The VOF model best predicts experimental HTC at high mass fluxes, although the results at low mass fluxes were predicted with reasonable accuracy. Based on the agreement of numerical and numerical results, the VOF model turned out to be a promising candidate for designing compact micro/mini channel heat exchangers.

Flow pattern-based heat transfer analysis of microchannel flow boiling: An experimental investigation

The present study investigates the flow boiling heat transfer and flow pattern of R245fa in a smooth, horizontal microchannel with an inner diameter of 1 mm. The experiments cover a range of mass flux from 300 to 700 kg/m2s and heat fluxes varying between 10 and 94 kW/m2, with corresponding saturation temperature of 23–54 °C. Heat transfer experiment results indicate that the local heat transfer coefficient (HTC) presents two distinct variation trends under different heat and mass flux conditions. In addition, the heat flux is positively correlated with the local HTC at each measure point. Visualization experiment results show that annular flow forms at low vapor quality and significantly influences heat transfer as the primary flow pattern. The flow boiling heat transfer of R245fa in the 1 mm microchannel can be characterized by three regimes: bubble/slug regime, liquid film evaporation regime, and intermittent dryout regime. Thin liquid film evaporation and intermittent dryout during annular flow predominately contribute to heat transfer. Finally, the experimental HTC is compared with five different predictive methods, revealing that the three-zone model proposed by Thome et al. [46] holds immense potential for estimating the local HTC in the microchannel.

Critical heat flux, heat transfer and pressure drop at high mass fluxes for R123 in a single microchannel

This paper presents new experimental data on pressure drop, heat transfer coefficient, and critical heat flux during flow boiling of low-pressure fluid R123 in microchannels. High mass velocities were used, above the values found in the literature in tests with this fluid. Experiments were conducted in a single smooth and horizontal microchannel, with an internal diameter of 1.1 mm and heating sections of 46 and 100 mm. The mass velocities used were between 1500 and 7500 kg/m2s. Saturation temperature ranged from 40.1 to 85.1 oC. The outlet vapor quality varied between -0.54 and 0.53 for the CHF results, between 0 and 0.5 for the pressure drop results, and between 0 and 0.35 for the HTC results, respectively. Record critical heat flux values for flow boiling of R123 in smooth microchannels were achieved, reaching 1.44 MW/m2. An analysis was made of the influence of an upstream pressure drop in avoiding instabilities during critical heat flux experiments. The highest heat transfer coefficient values during convective boiling of R123 in smooth microchannels were also obtained, with values close to 43.3 kW/m2K. The saturation temperatures used were among the highest for flow boiling of R123 in smooth microchannels found in the literature. Correlations for frictional pressure drop, heat transfer coefficient, and critical heat flux were evaluated in predicting the experimental data. An analysis was made on the influence of the void fraction correlation when estimating both the accelerational and frictional pressure drop.

Heat transfer characteristics of R134a flow boiling in a microfin tube under typical ORC pressures based on comparison with a smooth tube

The experimental investigation on the flow boiling heat transfer of an organic working fluid under typical organic Rankine cycle (ORC) operating conditions is insufficient, which directly affects the performance of the ORC system. In this paper, the flow boiling characteristics of R134a in a horizontal microfin tube were experimentally investigated in the reduced pressure range 0.6 ∼ 0.9. The general convection heat transfer characteristics of R134a flow boiling, along with the effects of heat flux, mass flux, pressure, and rewetting conditions, were investigated in a microfin tube and compared with that in a smooth tube. Results show that the main flow regime in the microfin tube under typical ORC operating pressures is the stratified flow with dry-out occurring at the top surface. The microfin effectively enhances the heat transfer and restrains the wall temperature separation through inducing strong helical flow due to buoyancy and centrifugal forces. With the heat flux increasing, the micro-fin strengthens the nucleate boiling by enhancing the vaporization cores, leading to more pronounced heat transfer enhancement compared with that in the smooth tube. The heat transfer on the bottom surface shows almost negligible sensitivity to variations in mass flux. However, in regions of high vapor quality, an increase in mass flux results in higher heat transfer coefficients on the top surface. Due to the interaction between intensified nucleate boiling and weakened gas convection when pressure is increased, it slightly affects the heat transfer coefficient. Unlike the smooth tube, the rewetting in the microfin tube can be maintained within a larger range of vapor quality due to the spiral flow caused by the micro-fin.