Increasing Vapor Quality Research Articles

Experimental investigation on flow boiling heat transfer of R290, R600a and their mixtures in a horizontal minichannel

The flow boiling heat transfer characteristics of R290, R600a and their mixtures were experimentally investigated in a horizontal minichannel with an inner diameter of 1.54 mm. In order to investigate the effects of mass flux and vapor quality, heat flux, saturation temperature, and component concentration on the flow boiling heat transfer coefficients, experiments were conducted with mass fluxes ranging from 200 to 400 kg/(m2·s), heat fluxes ranging from 20 to 30 kW/m2, saturation temperatures of 22 °C, 24 °C, and 26 °C, vapor qualities ranging from 0 to 1, and mass ratios of 70/30, 50/50, and 30/70 for R290/R600a. The experimental results indicate that with the increase of vapor quality, the mass flux promotes the heat transfer coefficient more obviously, and the heat transfer coefficient tends to increase first and then decrease. The heat transfer coefficients of both pure and mixed refrigerants increase with increasing the heat flux. The effect of saturation temperature on the heat transfer coefficients of pure and mixed refrigerants is not significant in this study. In the low vapor quality region, the heat transfer coefficients of R290/R600a mixtures are between those of R290 and R600a. As vapor quality increases, the heat transfer coefficients of the mixtures increase more slowly than those of pure refrigerants. In addition, experimental data were compared with the predictions of the empirical correlations. Among all the correlations, the Tran et al. correlation predicts the heat transfer coefficients for R290 and R600a best. The Lim and Kim correlation gives the best predictions of the heat transfer coefficients for R290/R600a mixtures.

Experimental analysis on the flow patterns and conversion mechanisms of condensing flow with non-azeotropic mixtures in spiral tube

The flow patterns have a significant impact on the flow and heat transfer characteristics of the working fluid, making it fundamental for the study of complex two-phase flows. To investigate the condensation flow pattern and flow pattern transformation mechanism with mixed hydrocarbon in a spiral tube, a two-phase flow pattern experimental system was designed. The effects of mass flux (196–540 kg/m−2·s−1), vapor quality (0–1), and operating pressure (2–4 MPa) on flow patterns of methane/ethane/propane/isobutane mixed fluid in spiral tubes were analyzed. The results showed that with the increase in vapor quality, flow patterns such as bubbly flow, intermittent flow, wavy-stratified flow, and annular flow were observed in sequence. Additionally, through a comparative analysis of the experimental observations with existing flow pattern maps, a new flow pattern map tailored for the condensation two-phase flow of mixed hydrocarbon working fluids has been established. Based on the influence of inertial force, surface tension, gravity, shear force and other forces, Martinelli number, Soliman We and Soliman Fr are selected for the development of flow pattern conversion criteria. The new flow pattern map accurately predicts the majority of flow patterns.

An Experimental Investigation of R600a Condensation in a Multiport Microchannel.

This study aims to provide condensation heat transfer coefficients of R600a (isobutane) refrigerant under mass fluxes between 50 and 98 kg/m2·s at saturation temperatures of 35 °C, 40 °C and 45 °C. Additionally, experiments are conducted with varying inlet vapour quality to understand its effect on the condensation heat transfer measurement. An aluminium multiport microchannel with a hydraulic diameter (Dh) of 0.399 mm is used, where a plexiglass cover is mounted on the top of the microchannels to observe the flow conditions. A 1D heat transfer through the aluminium block is assumed, and heat flux through the refrigerant to the coolant is measured to obtain condensation heat transfer coefficients of R600a. The results showed that decreasing saturation temperature and increasing vapour quality increase the condensation heat transfer coefficient. Increasing refrigerant mass flux increases the heat transfer coefficient up to a specific mass flux. It is observed that the effect of inlet vapour quality becomes significant as introduced quality decreases due to increasing fluctuation.

Flow boiling heat transfer of R1234ze(E) in a horizontal mini-channel at medium and high saturation temperatures

This study reports experimental results of flow boiling heat transfer characteristics of R1234ze(E) (a low global warming potential refrigerant) at moderate and high saturation temperatures. The study was performed in a single horizontal circular smooth tube of 2.07 mm internal diameter. Heat transfer characteristics and flow pattern visualization through high-speed camera were investigated for saturation temperatures ranging from 60 to 95 °C, corresponding to reduced pressures from 0.35 to 0.75, mass velocities from 300 to 500 kg/m2s, heat fluxes from 30 to 50 kW/m2 and vapor qualities from 0 to 1. The pre-dryout data (214 data points) measured were compared against different prediction methods from the literature developed under different theories. The study revealed that the saturation temperature has a significant effect on heat transfer coefficient whereas the mass velocity presented a less pronounced or negligible effect. Furthermore, the heat transfer coefficient was observed to decrease slightly to a minimum in the low vapor quality region and then levels off or increases slightly with increasing vapor quality until dryout occurs. As a result, nucleate boiling was observed to be the dominant mechanism controlling the heat transfer. It was observed that the higher the saturation temperature, the earlier the vapor quality at which dryout incipience occurs. Considering the 214 pre-dryout data points analyzed, the methods proposed by Kew and Cornwell (1997), Lazarek and Black (1982) and Choi et al. (2007) were the most accurate, predicting the entire pre-dryout database with mean absolute errors (MAEs) of 13.7%, 13.9 % and 14.4 %, respectively.

Effects of Heat Transfer Characteristics of R32 and R1234yf with Al2O3 Nanoparticle through U-Bend Tube Evaporator

This study used the Ansys Fluent® computational fluid dynamics code in conjunction with a volume of fluid multiphase model and phase-change model to analyze the flow boiling evaporation heat transfer coefficient and flow patterns of R32 and R1234yf with Al2O3 nanoparticle through the U-bend tube with a curvature ratio for downward-oriented flow. The volume of fluid (VOF) model was used to follow the patterns at the interface, while the SST k-omega model was used to simulate the gas-liquid flow. This work has been validated by utilizing a R134a refrigerant. Simulations were performed at various mass fluxes, vapor qualities, and temperatures to determine the effects of these variables on heat transfer and frictional pressure decrease in the tube. R1234yf shows much better performance than the other pure refrigerants in terms of heat transfer and vaporization. The addition of nanoparticle Al2O3 with the refrigerants R32 and R1234yf significantly improved the heat transfer coefficient and increased the vapor fraction. The frictional pressure drop increases with increasing mass flux and decreases with increasing vapor quality due to a significant decrease in the liquid film thickness. The heat transfer coefficient, on the other hand, increases with increasing mass flux and decreases with vapor quality up to a point. There are certain changes in the heat transfer coefficient at the bend. After the bend, the frictional pressure drop increased at a higher rate than before the bend, and the vapor fraction increased at a higher rate.

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%.

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.

Jet impingement boiling heat transfer performance of refrigerant HP-1 in micro-pin-finned surfaces for high-power chips

In this paper, jet impingement boiling heat transfer performance was studied using a novel new environmentally friendly refrigerant medium (HP-1) based on a high-power chip heat sink. A series of experimental investigations of jet impingement boiling were conducted on smooth and micro-pin-finned surfaces (PF0.5–0.5–1, PF0.5–0.5–2 and PF0.3–0.3–1). Heat transfer performances were evaluated over the saturated pressures from 450 to 590 kPa, liquid subcoolings from 2 to 15 K, nozzle sizes from 1 to 2 mm, mass fluxes from 71 to 858 kg/(m2s). The results show that critical heat flux (CHF) and heat transfer coefficient (HTC) increases with the increase of mass flux and saturated pressure. At ΔTsub=15 K, the CHF of the micro-pin-finned surfaces can be increased by 63 % compared with smooth surface, in which the CHF of PF0.5–0.5–2 surface is up to 1532 kW/m2. Boiling heat transfer coefficient first increases and then decreases with the increase of heat flux. Due to the increase of the inlet liquid subcooling, leading to a large temperature difference, so the heat transfer coefficient is smaller at a high liquid subcooling. Several factors, such as mass flux, saturated pressure and liquid subcooling, have a great impact on pressure drop. With the increase of vapor quality, the shear action between the vapor-liquid phase is enhanced, and the pressure drop will gradually increase. Bubble behaviors have also been captured by a high-speed camera to help analyze the mechanism of jet impingement boiling heat transfer in a confined space for high-power chips.

Flow condensation inside a multiport mini channel and a rectangular mini channel with pin fin array

In this paper, an experimental study of flow condensation was carried out of R134a in a multiport mini channel and a mini channel with pin fin array. The former comprised 20 parallel rectangular channels with an equivalent diameter of 0.64 mm. The latter is a narrow rectangular mini channel containing 10 rows of diamond pin fins with a staggered configuration, and height and longitudinal/transverse pitch of 0.5 mm and 2 mm respectively. To acquire the local value of heat flux and heat transfer coefficient, air jet impingement cooling devices were adopted to condense the vapor of refrigerants. The effects of vapor quality, mass flux, heat flux, and saturation pressure on flow condensation heat transfer coefficient were investigated with the operating conditions: vapor quality from 1 to 0, mass flux from 160 to 450 kg/(m2s), heat flux from 10.0 to 39.8 kW/m2, and saturation pressure from 600 to 1500 kPa. The experimental results indicated that the pin fin array significantly improves the condensation heat transfer coefficient up to nearly 186 % compared to the multiport mini channel in the high vapor quality region. For both channels, the local heat transfer coefficient increases with an increase in vapor quality, mass flux, and heat flux whereas decreases with increase in saturation pressure. The influence of heat flux and mass flux on heat transfer coefficient was more pronounced in the high vapor quality region than in the low vapor quality region. A sensitivity analysis was performed at different vapor quality levels. The most influential parameters in the smooth channel and the pin fin array are saturation pressure and mass flux, respectively. The relative contribution of heat flux is more obvious in the high vapor quality region. Some of the existing correlations have good prediction performance for the smooth channel experimental data in this paper, and the corresponding MAD is within 20 %. However, their deviations are much greater for the applications of pin fin array.

Analysis of influence of inlet vapor quality on heat transfer and flow pattern in mini-channels during flow condensation process

Micro/mini-channel heat sinks are extensively utilized in heat dissipation systems that involve high heat flux components. These heat sinks are preferred due to their compact size and excellent heat transfer efficiency. The selection of a refrigerant condensation system for the heat dissipation system under vapor conditions is crucial. In order to investigate the heat transfer characteristics of flow condensation in a mini-channel, a visual experiment system was used with a hydraulic diameter of 2 mm and a volume flowrate of 30 mL/min, 60 mL/min, 90 mL/min, 120 mL/min respectively. The study focused on the effect of inlet vapor quality on the flow pattern and heat transfer characteristics of flow condensation in the mini-channel. The results indicate that the flow pattern in flow condensation process can be classified into annular flow, transition flow, slug flow and bubble flow in sequence along the flow direction. As the inlet vapor quality (xin) increases, the dominance of annular flow gradually increases, making it difficult to capture the bubble flow with high volume flowrate and high inlet vapor quality. This phenomenon is observed under experimental conditions when: 1) xin is equal to or greater than 0.75 at a volume flowrate of 60 mL/min, 2) xin is equal to or greater than 0.55 at a volume flowrate of 90 mL/min, and 3) xin is equal to or greater than 0.25 at a volume flowrate of 120 mL/min. The increase in inlet vapor quality is beneficial to the improvement of condensation heat transfer coefficient. In this experiment, it was observed that the local heat transfer coefficient was decreased by up to 83.7 % when comparing the lowest vapor quality to the highest at the same volume flowrate, emphasizing the significance of inlet vapor quality in the flow condensation during the heat transfer process.

Experimental study on heat transfer and flow pattern transition for R1233zd/FC72 mixtures across tube bundles

An experimental investigation was conducted to study the heat transfer and two-phase flow patterns of pure R1233zd and non-azeotropic R1233zd/FC72 mixtures (0.75/0.25, 0.5/0.5, and 0.25/0.75 by mass) across tube bundles. The test section of tube bundles was transparent to observe the flow pattern. The flow boiling heat transfer was measured. The experiments were carried out with vapor qualities of 0.1–0.9, mass flow rates of 3–5 kg/h and heat fluxes of 3000–7000 W/m2. Falling film flow, bubbly flow, bubbly-shear flow, and shear flow were observed. The flow pattern transition maps were presented for various conditions. Increasing the mass fraction of the volatile component R1233zd causes the flow pattern transitions occurring at lower vapor qualities. The heat transfer coefficient of mixtures is lower than that of pure R1233zd. With increasing vapor quality, the heat transfer coefficient increases for both pure R1233zd and the mixture with a R1233zd mass fraction of 75 %. However, for mixtures with R1233zd mass fractions of 50 % and 25 %, the heat transfer coefficient initially increases and then decreases with increasing vapor quality. For vapor qualities higher than 0.5, increasing the volatile component, R1233zd, significantly enhances heat transfer. The heat transfer coefficient increases with increasing mass flow rate and heat flux. The experimental data were compared with correlations in the literature with poor agreement. An improved heat transfer correlation for non-azeotropic mixtures flowing across tube bundles was proposed, which effectively predicted the experimental data.

Saturated flow boiling heat transfer of R1233zd(E) in parallel mini-channels: Experimental study and flow-pattern-based prediction

The parallel channel is widely used in the two-phase flow boiling heat transfer due to its simple structure, but the existence of multiple flow patterns causes a complex and non-unified explanation of the heat transfer mechanisms. In this study, a saturated flow boiling experiment was designed in a parallel mini channel heat sink with R1233zd(E) as the working fluid. By controlling the vapor quality difference at the inlet and outlet to around 0.1–0.2, each single flow pattern was isolated and presented in the test section, and the corresponding variations of heat transfer coefficient (HTC) were discussed. The results showed that the flow pattern during the saturated flow boiling included bubbly flow, slug flow, wavy-annular flow and local dry-out. The transition boundaries between bubbly flow, slug flow and wavy-annular flow showed good agreement with the present criteria, while a modification was made on the transition from wavy-annular flow to local dry-out. The HTC showed a fast increase under low vapor qualities, then became stable and decreased with the increase of vapor quality. In addition, 6 correlations were selected to compare with the measured HTC classified according to the flow patterns. The comparison showed that the present correlations had different prediction accuracies on the HTC data under each flow pattern. The best prediction on the heat transfer during bubbly and slug flow was achieved by Liu and Winterton's model with the MAE of 16.4% and 12.9%, while Li and Wu's model showed the best agreement with the HTC data during wavy-annular flow with the MAE of 9.1%.

Pressure drop characterization on cryogenic multiphase flow in spiral wounded heat exchanger under high mass flow conditions

In order to clarify the influence of dryness, working medium and mass flow rate on pressure drop characteristics in shell side of spiral wound heat exchanger, a simplified test section has been analyzed in this study. The model is with a coiled section under compact design: 1 mm layer spacing of and 2 mm tube spacing in the main structure. The mixed alkane composed of methane, ethane, propane and others are used as working fluid. The current study is focused on the pressure drop behaviors and the effects of multiphase flow in different stages of cooling process in the cryogenic heat exchanger. Systematic numerical model has been established in this study to investigate the pressure behaviors, which are also verified against newly constructed experimental system. Operating conditions range from vapor quality of 0.1–0.9, mass flux of 60–120 kg·(m2·s)−1, under the operation pressure of 0.25 MPa and temperature from −27 °C to −170.6 °C. The absolute error of pressure measurement is less than 1.5 kPa and the absolute error of pressure drop measurement is less than 25 Pa, and the relative error is 0.25 % for the current test. The experimental results show that the friction pressure drop increases with the increase of vapor quality from 0.1 to 0.9 and mass flux from 60 to 120 kg·(m2·s)−1, and the growth gradient of friction pressure drop decreases with the increase of vapor quality. A set of pressure drop correlation has been established to reflect the influence of factors such as vapor quality and mass flow rate, and the deviation between predicted pressure drop and experimental data was within ±20 %.

Characteristic of heat transfer degradation during flow boiling of low GWP refrigerant R1336mzz(Z) in an enhanced channel

The decreasing heat transfer coefficient with an increase in vapor quality, which is termed heat transfer degradation, commonly exists during flow boiling in channels. Experimental investigations have been conducted on the characteristics of heat transfer degradations for a low GWP refrigerant R1336mzz(Z) in an enhanced channel. The heat transfer degradations occur in both low mass flux and high mass flux. In the analysis of the mechanisms for heat transfer degradations, the effects of heat flux and saturation temperature are taken into account. Additionally, a comparison with the heat transfer characteristics of another refrigerant (R245fa) with different thermophysical properties is also included to reinforce the analysis. The heat transfer degradation at low mass flux is accelerated by increasing the heat flux or saturation temperature, and R245fa shows an earlier heat transfer degradation than R1336mzz(Z). However, a completely reverse phenomenon appears at high mass flux. The heat transfer degradation can be delayed or even eliminated as the saturation temperature and heat flux increase, and the degradation does not show up for R245fa under the same operating conditions. The mechanisms of heat transfer degradations at low mass flux and high mass flux are explored to be different. Dryout is the cause of heat transfer degradation at low mass flux, whereas suppression of nucleate boiling is the inducement at high mass flux.

Experimental study of steam condensation heat transfer characteristics in a horizontal tube under rolling motion

As an efficient heat transfer method, in-tube condensation heat transfer is widely used in various fields, including passive residual heat removal systems (PRHRS) of floating power plants under ocean conditions. The experimental investigation aimed to study the condensation heat transfer characteristics of steam in a 25 mm inner diameter horizontal tube with a heat transfer length of 1050 mm under rolling motion. The study used a mass flux of 30 kg/(m2·s) and investigated average vapor qualities, rolling amplitude, and rolling period ranging from 0.32 to 0.79, 0 °–20°, and 10 s–20 s, respectively. The experimental results show that the condensation flow patterns under rolling conditions are more complex than those under non-rolling motion due to the influence of the rolling period and rolling amplitude. The instantaneous heat transfer coefficient (IHTC) changes periodically due to the rolling motion and has the same change period. The pulsation amplitude of the IHTC increases with increasing rolling amplitude but weakens as the vapor quality increases. The effect of the rolling period on the IHTC is much less than that of the rolling amplitude. At low vapor quality, the time-averaged heat transfer coefficient (THTC) under the rolling motion condition increases with the rolling amplitude and period, and the maximum enhancement of THTC is 44%. As the vapor quality increases, the THTC under rolling conditions is the same as that under non-rolling conditions. These findings provide important insights into the condensation heat transfer characteristics of vapor in a horizontal tube under rolling motion, which can help improve the design and operation of in-tube condensation heat transfer systems in various fields.

Evaluation of the gravitational effects on flow boiling heat transfer of R134a in a 0.5 mm tube under conditions of imposed wall-temperature

This paper presents an experimental investigation of heat transfer coefficient and dryout inception during flow boiling of R134a in a circular channel with an internal diameter of 500 μ m for horizontal, 45° inclined, and vertical upward flows. The microchannel was heated by imposing the wall temperature by flowing hot water counter-currently to the test fluid through an annular region containing the test section. Experiments were performed for mass velocities ranging from 350 to 600 kg/(s·m2), heat fluxes up to 46 kW/m2, saturation temperature of 30 °C, and vapor qualities from 0 to 1. The experimental data were parametrically analyzed, and the effects of the experimental parameters (heating method, mass velocity, heat flux, and channel orientation) were identified. Almost similar heat transfer behaviors were found under conditions of imposing wall temperature and heat flux through the Joule effect. The effect of the flow orientation on the heat transfer coefficient prior to the dryout inception was negligible. The heat transfer coefficient increases with increasing vapor quality, heat flux, and mass velocity. The critical heat flux exhibited similar values independently of the flow orientation. Moreover, the critical heat fluxes increases as the dryout inception vapor quality reduces, regardless of the flow orientation.

R245fa flow boiling heat transfer in a sintering and electroplating modulated tube

• R245fa flow boiling heat transfer is investigated in a modulated tube coated by using sintering and electroplating technology. • Flow patterns and dynamic liquid height analyses are proposed to explore the heat transfer enhancement mechanisms. • The micro-nano scale coating on the tube inner wall could modulate flow pattern to enhance heat transfer. • The maximum EF and PEC can reach 2.87 and 2.79. The flow boiling performance of R245fa was investigated in a bare stainless-steel tube (BT) and a sintering and electroplating modulated stainless-steel tube (SET). The test tubes had the same inner diameters of 10.02 mm and effective heat transfer lengths of 800 mm. The mass fluxes, inlet vapor mass qualities and heat fluxes were in ranges of 192.61-705.35 kg/(m 2 ·s), 0.01–0.9 and 4.99-74.97 kW/m 2 , respectively. Three flow patterns were identified in both BT and SET. The annular flow pattern conversion line of SET was earlier than that of the BT. With the increase in vapor quality, the boiling heat transfer coefficient increased and then decreased at q≤ 45 kW/m 2 , whereas it decreased at q ≥60 kW/m 2 . For the BT, the heat transfer coefficient was weakly affected by the mass flux, but it increased with the increase in mass flux for the SET. For the BT and SET, the frictional pressure drop increased and then decreased with the increase in mean vapor quality at a certain heat flux. Owing to the strong wettability of SET, the thick liquid film at the tube bottom in stratified flow was forced to the tube top and the liquid droplets in annular flow were captured by the surface coating. Thus, flow patterns were modulated to enhance heat transfer. The maximum of the heat transfer enhancement ratio and performance evaluation parameter can reach 2.87 and 2.79, respectively.

EXPERIMENTAL INVESTIGATION OF TWO-PHASE PRESSURE DROP IN THE STRAIGHT ADIABATIC TUBES

The pressure drop is the significant constraint in designing the single or multiphase flow systems. Innovative technologies, such as nuclear power plants and enhanced oil recovery processes, can be designed more efficiently with the knowledge of the multiphase pressure drop phenomenon. The higher pressure drop induces the lower system efficiency. There is a scarcity of literature representing the steam-water flow at atmospheric system pressure in the adiabatic tubes. The experiments are performed to get the two-phase pressure drop in the adiabatic tube for the effective design of heat transfer equipment used for diverse applications. The adiabatic tubes of 8, 13.7, and 18 mm diameter, having 15 × 10<sup>2</sup> mm length, are experimented for 32-495 kg/m<sup>2</sup>s mass flux and 0-1 vapor quality. The pressure drop in the adiabatic tubes increases nonlinearly with the increase of vapor quality. The pressure drop is influenced significantly by the diameter of the tube. The pressure drop is lesser in the 18 mm tube compared to the 8 and 13.7 mm tubes for constant vapor quality and mass flow rate. The increase of the mass flux increases the pressure drop monotonically. The pressure drop is higher at higher mass flux and higher vapor quality in the lesser diameter tube. The experimental pressure drop is compared to various correlations. A correlation is suggested to measure the two-phase pressure drop during steam-water flow at atmospheric system pressure in the adiabatic tubes.

Numerical simulation on shell-side flow pattern transition and heat transfer of non-azeotropic refrigerant mixture

The non-azeotropic hydrocarbon refrigerant mixture flowing across shell-side of spiral wound heat exchanger is widely used in liquefied natural gas production industry. However, the phase-change heat transfer mechanism is still lack of thorough understanding. A numerical model is presented for simulating evaporation heat transfer of non-azeotropic hydrocarbon refrigerant mixture, CH4/C2H6, across multiple spiral wound tubes. Volume of Fluid model (VOF) was used to obtain liquid–vapor interface. The heat and mass transfer rates for each component were given as source terms. Thermophysical properties of the mixture were given using mixing laws in the literature. A flow pattern map was developed based on simulation results. The flow pattern transition and heat transfer were analyzed for the mixture with several component mixing ratios. With increasing vapor quality, pure ethane and methane-ethane mixture appear different flow patterns. The pure ethane has smoother liquid film on the tube wall than the methane-ethane mixture. The increasing of methane promotes the flow pattern transition. With vapor quality of 0.2–0.3, falling film flow transfers to shear flow. Shear flow transfers to spray flow for vapor quality of 0.7–0.8, where the heat transfer coefficient starts to decrease for methane-ethane mixture. For falling film flow, the heat transfer coefficient of pure ethane is higher than the methane-ethane mixture. The outcomes may help spiral wound heat exchanger design and evaluation for liquefied natural gas production.

Heat transfer and pressure drop characteristics of R1234yf during evaporation in a plate heat exchanger with offset strip fins: An experimental study

Plate heat exchangers (PHEs) have been suggested for use in secondary loop air conditioning systems in automobiles operated with flammable refrigerants, such as R1234yf. In this study, the evaporation heat transfer coefficient (HTC) and two-phase frictional pressure drop (FPD) of R1234yf in a brazed PHE with offset strip fins were investigated. The mass flux in the experiment was 40–80 kg m−2 s−1, heat flux was 4–10 kW m−2, vapor quality at the inlet was 0.1–0.8, and saturation temperature was 5–15°C. Prior to the primary analysis, a coolant-to-coolant test was performed using a modified Wilson plot approach to estimate the multiplier (C) and the Reynolds number exponential (n). The observed C and n values were 0.868 and -0.496, respectively, with an R2 value of 0.977. The main results demonstrated an increase in the evaporation HTC with an increase in vapor quality in the initial stages, followed by a subsequent decrease, forming a peak at a mean vapor quality of 0.35–0.41. The nucleate and convective heat transfer regimes were dominant during the evaporation of R1234yf at lower and higher mean vapor qualities, respectively. Evidently, the evaporation HTC was significantly influenced by the mass flux and vapor quality compared with the heat flux and saturation temperature. Furthermore, the two-phase FPD continuously increased with the mass flux, vapor quality, and heat flux, although it decreased with the saturation temperature. As a result, new correlations are proposed herein for forecasting the Nusselt number and friction factor of R1234yf with average absolute errors of ±12% and ±15%, respectively.