Horizontal Smooth Tube Research Articles

Experimental flow boiling characteristics of R134a/R245fa mixture inside smooth horizontal tube

The flow boiling characteristics were experimentally studied in smooth horizontal tube for pure R245fa and one of its mixtures, R134a/R245fa with 0.82/0.18 in mass fraction, which was regarded as a promising working fluid for Organic Rankine Cycle (ORC). The flow boiling heat transfer coefficient and pressure drop for the two fluids were measured under different vapor quality, mass flux, tube wall heat flux and evaporating pressure. Correlation for predicting the flow boiling heat transfer coefficient was developed for the selected mixture based on the experimental data. The results indicated that the flow boiling heat transfer coefficients of the two working fluids went up with mass flux, heat flux and evaporating pressure under the experimental conditions. The mixture of R134a/R245fa with 0.82/0.18 in mass fraction had less pressure drop, but higher heat transfer coefficient than that of the pure R245fa. The absolute errors of 89% experimental data are within 10% of prediction of the present developed correlation for the mixture. The results may benefit the potential utilization of R134a/R245fa in ORC.

Heat transfer correlation of the falling film evaporation on a single horizontal smooth tube

The falling film heat transfer of R134a outside a single horizontal smooth tube is experimentally investigated, and the effects of the tube diameter, saturation temperature, film flow rate and heat flux are studied. A threshold Reynolds number is proposed to delineate the test data into full wetting and partial dryout regimes. New correlations based on the present data and some data in literature are fitted for both regimes. The correlation for partial dryout regime fits 91% of the 153 data within ±20%, and the correlation for full wetting regime fits 94% of the 205 data within ±20%. The correlations have also been compared with previous measured data of other refrigerants available in literature. It is found that the predictions for partial dryout regime agree with most of the previous data with a deviation of ±30%.

Experimental study of boiling heat transfer in smooth/micro-fin tubes of four refrigerants

An experimental investigation of boiling characteristics in a horizontal smooth and micro-fin tube with 9.52mm outside diameter and 1m length was conducted. The refrigerants tested were R22, R134a, R407C and R410A while vapour quality ranges from 0.1 to 0.9, mass flux 50, 250, 450kgm−2s−1 and heat flux of 5, 12.5, 20kWm−2. The saturation temperature is 5°C. For the smooth tube, the average heat transfer coefficients of R134a, R407C and R410A are 110.9%, 78.0% and 125.2% of those of R22 in test conditions respectively. For the micro-fin tube, the average heat transfer coefficients of R22, R134a, R407C and R410A are 1.86, 1.80, 1.69 and 1.78 times higher than those of the smooth tube. The pressure drop of R22, R407C and R410A for the smooth tube is similar to each other while the pressure drop of R134a is 1.7 times higher. The average pressure drop of R22, R134a, R407C and R410A for the micro-fin tube is 1.42, 1.30, 1.45 and 1.40 times higher when compared with that for the smooth one. Considering the effect of heat transfer enhancement and pressure drop augment, the efficiency index η1 which values the thermo-hydraulic performance at identical flow rate of R22, R134a, R407C and R410A in the micro-fin tube used is 1.31, 1.38, 1.17 and 1.27 respectively compared with the smooth tube.

An experimental study of flow boiling frictional pressure drop of R134a and evaluation of existing correlations

A series of experiments on flow boiling frictional pressure drop of R134a in three horizontal circular smooth copper tubes with inner diameters of 1.002, 2.168, and 4.065mm were conducted. A total of 397 experimental data points are obtained for a mass flux range of 185–935kg/m2s, a heat flux range of 18.0–35.5kW/m2, a saturation pressure range of 0.578–0.82MPa, and a vapor quality range of 0.03–1.0. The effects of mass flux, heat flux, vapor quality, saturation pressure, and diameter on the frictional pressure drop are analyzed and the results indicate that the frictional pressure drop increases with increasing mass flux, increases with vapor quality until a peak and then drops, decreases with increasing saturation pressure and hydraulic diameter, and almost keep constant with heat flux. Thirty-two existing correlations of two-phase frictional pressure drop are compared with the experimental data. The best correlation has the mean absolute deviation of 15.0%. Besides, an experimental database of flow boiling frictional pressure drop is complied, which contains 3244 data points, including 397 from the current experiments and 2847 from 27 published papers. The 32 existing correlations are evaluated with the database. The best correlation has the mean absolute deviation of 24.5%. The work provides a guide for selecting a suitable correlation for specific applications of flow boiling frictional pressure drop.

Numerical study of nanofluids condensation heat transfer in a square microchannel

ABSTRACTThis paper presents a numerical study of nanofluids condensation heat transfer inside a single horizontal smooth square tube. The numerical results are compared with the previous experimental predictions. The numerical results show that the heat transfer coefficient could be improved within 20% by increasing the volume fraction of Cu nanoparticle by 5% or by increasing the mass flux from 80 to 110 kg/m2 s. Reducing the hydraulic diameter of the microchannel from 200 to 160 µm leads to an increase in the condensation average heat transfer coefficient by 10%. A new correlation estimating the Nusselt number for the condensation of nanofluids or pure vapor is proposed. It predicts average condensation heat transfer with a good agreement with those computed.

Saturated flow boiling of HFC134a and its low GWP substitute HFO1234ze(E) inside a 4 mm horizontal smooth tube

This paper presents the comparative analysis of HFO1234ze(E) and HFC134a during vaporisation inside a 4 mm smooth tube. The experimental tests were carried out at three different saturation temperatures (10, 15, and 20 °C) at increasing vapour quality up to incipient dryout to evaluate the specific contribution of heat flux, refrigerant mass flux, mean vapour quality, and saturation temperature (pressure). The heat transfer coefficients have a positive slope versus vapour quality and the slope increases with refrigerant mass flux and decreases with heat flux. Saturation temperature (pressure), refrigerant mass flux and mean vapour quality have a remarkable impact on the frictional pressure drop of both HFO1234ze(E) and HFC134a whereas the effect of heat flux appears marginal or negligible. Convective boiling seems to be the prevailing heat transfer regime in the present experimental tests. HFO1234ze(E) exhibits heat transfer coefficients similar to HFC134a and slightly higher frictional pressure drops.

冷媒R245faの水平平滑管内凝縮熱伝達および圧力損失-凝縮特性に及ぼす飽和温度の影響

冷媒R245faの水平平滑管内凝縮熱伝達および圧力損失-凝縮特性に及ぼす飽和温度の影響

Comparative assessment of condensation and pool boiling heat transfer on horizontal plain single tubes for R1234ze(E), R1234ze(Z), and R1233zd(E)

The low-global warming potential (GWP) refrigerant of R1234ze(E) is considered to be a prospective alternative to conventional R134a for car air conditioners, whereas the isomer R1234ze(Z) and R1233zd(E) are anticipated to be the low-GWP alternatives to R245fa used in high-temperature heat pumps and organic Rankine cycles. The free convective condensation and pool boiling heat transfer coefficients (HTCs) of these refrigerants on a horizontal smooth tube made of copper with an outer diameter of 19.12 mm are comparatively assessed in this study. For the condensation experiment, the measured HTCs of R1234ze(E) and R1234ze(Z) agree with the theoretical HTC within ±10%. However, the experimental HTC of R1233zd(E) is biased by +25% on average to the theoretical HTC. This suggests the considerable estimation error in R1233zd(E) transport properties. From the pool boiling observations, the role of surface tension on nucleate boiling was identified by the bubble behavior. The experimental boiling HTC closely agrees with the HTC calculated by Ribatski and Jabardo (2003).

The effect of oscillating flow on a horizontal dilute-phase pneumatic conveying

ABSTRACTA horizontal dilute-phase pneumatic conveying system using vertically oscillating soft fins at the inlet of the gas–particle mixture was studied to reduce the power consumption and conveying velocity in the conveying process. The effect of different fin lengths on horizontal pneumatic conveying was studied in terms of the pressure drop, conveying velocity, power consumption, particle velocity, and intensity of particle fluctuation velocity for the case of a low solid mass flow rate. The conveying pipeline consisted of a horizontal smooth acrylic tube with an inner diameter of 80 mm and a length of approximately 5 m. Two types of polyethylene particles with diameters of 2.3 and 3.3 mm were used as conveying materials. The superficial air velocity was varied from 10 to 17 m/s, and the solid mass flow rates were 0.25 and 0.20 kg/s. Compared with conventional pneumatic conveying, the pressure drop, MPD (minimum pressure drop), critical velocities, and power consumption can be reduced by using soft fins in a lower air velocity range, and the efficiency of fins becomes more evident when increasing the length of fins or touching particles stream by the long fins. The maximum reduction rates of the MPD velocity and power consumption when using soft fins are approximately 15% and 26%, respectively. The magnitude of the vertical particle velocity for different lengths of fins is clearly lower than that of the vertical particle velocity for a non-fin conveying system near the bottom of the pipeline, indicating that the particles are easily suspended. The intensities of particle fluctuation velocity of using fins are larger than that of non-fin. The high particle fluctuation energy implies that particles are easily suspended and are easily conveyed and accelerated.

Hydrocarbon condensation in horizontal smooth tubes: Part I – Measurements

This paper presents the results from an experimental investigation of propane condensing inside horizontal tubes at three mass fluxes, two tube diameters and reduced pressures of 0.25–0.95. These conditions represent the saturation pressures and mass fluxes of most interest to the process and HVAC&R industries. In this first part, Part I, of a two-part study, the trends in heat transfer coefficient and frictional pressure gradient for these conditions are discussed and comparisons with available models from the literature are made. A small increase in heat transfer coefficient with tube diameter is observed for the specific tube diameters range studied, which is different from the trend predicted by most correlations. High-speed flow visualization results from a previous study, conducted using the same test facility, are used to explain the trend with tube diameter. No heat transfer coefficient correlation from the literature predicted all the results from the present study adequately; notably, all correlations perform increasingly poorly at higher reduced pressures. Also, no correlation from the literature adequately predicts the frictional pressure drops measured in this study. In the companion paper, Part II, the heat transfer and pressure drop results from this paper are used to develop correlations applicable to a wide range of operating conditions.

An experimental study of flow boiling heat transfer of R134a and evaluation of existing correlations

A series of experiments on flow boiling heat transfer coefficient of R134a in three horizontal circular smooth copper tubes with inner diameters of 1.002, 2.168, and 4.065mm were conducted. A total of 397 experimental data points are obtained for a mass flux range of 185–935kg/m2s, a heat flux range of 18.0–35.5kW/m2, a saturation pressure range of 0.578–0.82MPa, and a vapor quality range of 0.03–1.0. The effects of heat flux, saturation pressure, vapor quality, mass flux, and diameter on the heat transfer coefficient are analyzed and the results indicate that the heat transfer coefficient is higher for smaller tubes and for higher saturation pressure while the effects of heat flux, mass flux, and vapor quality are different for different tubes. There are 28 existing correlations of flow boiling heat transfer compared with the experimental data, and the comparison results indicate that the best correlation has a mean absolute deviation of 12.1% and has strong ability to trace the tendency of the heat transfer coefficient with vapor quality. Besides, an experimental database of R134a flow boiling heat transfer is complied, which contains 3000 data points, including 397 from the current experiments and 2603 from 21 published papers. The 28 existing correlations are evaluated with the database, providing a guide for selecting a suitable correlation for specific applications of R134a flow boiling heat transfer.

Hydrocarbon condensation in horizontal smooth tubes: Part II – Heat transfer coefficient and pressure drop modeling

In this second part, Part II of a two-part study, data reported in Part I for condensation of hydrocarbons over a wide range of conditions are used to develop models for pressure drop and heat transfer coefficients. The pressure drop model accounts for the pressure drop occurring in the vapor and liquid phases, and due to phase interactions. The model provides improved predictions over existing correlations at lower frictional pressure drop conditions, such as in larger tube diameters and at higher saturation pressures. The data from Part I of this study are also used to develop a physically consistent condensation heat transfer model for horizontal smooth tubes. This model incorporates a novel method to account for the heat transfer through the upper liquid film that typically forms in horizontal tubes. The approach models the evolution of the liquid film from purely horizontal (annular) flow to a combination of vertical (falling film) and horizontal (annular) flow. Moreover, the evolution with quality is modeled continuously without resorting to interpolation between flow regimes. In doing so, this model accurately reflects the observations from flow visualization studies, and is a departure from the approach typically followed in existing correlations. Both models are developed using a database containing only hydrocarbon data (propane and pentane), but are shown to also accurately predict synthetic refrigerant behavior. Special attention is paid to developing a model that is equally applicable to all of the operating conditions in the database, thus equal weighting is given to high pressure and low pressure conditions, and all mass fluxes and tube diameters. The models predict the frictional pressure drop and heat transfer coefficient for the propane database with average deviations of 3% and −1%, respectively.

Flow regimes and void fractions during condensation of hydrocarbons in horizontal smooth tubes

Phase change in hydrocarbons is of interest to the petroleum, refining, and chemical processing industry, as well as in HVAC and in organic Rankine power cycles. An experimental investigation of flow regimes and void fractions during condensation of propane flowing through smooth horizontal tubes with internal diameters of 7.0mm and 15.0mm was conducted. Measurements were made over mass fluxes ranging from 75 to 450kgm−2s−1, operating pressures ranging from 952 to 1218kPa, and nominal vapor qualities ranging from 0.05 to 0.95. Results from this experimental study are compared with predictions from correlations in the literature, and agreement and differences are interpreted and discussed. Appropriate flow regime transition criteria between the wavy and annular flow regimes are highlighted. Detailed analyses of the video frames are also used to develop a new multi-regime void fraction model that uses the drift flux model framework. The model provides improved agreement with the experimental results when compared to correlations from the literature, and predicts the trends with mass flux, tube diameter and pressure well.

Experimental investigation of flow boiling heat transfer and pressure drops characteristic of R1234ze(E), R600a, and a mixture of R1234ze(E)/R32 in a horizontal smooth tube

The saturated flow boiling heat transfer coefficients and frictional pressure drops characteristics of R1234ze(E), R600a, and L-41b (R1234ze(E)/R32 (27/73 mass %)) inside an 8 mm inner diameter horizontal tube were investigated. The experiment was carried out at the saturation temperature of 20°C with heat flux ranging from 5.0 to 10.0 kW·m−2 and mass flux ranging from 200 to 400 kg·m−2·s−1. The influence of mass flux, heat flux, and quality on the heat transfer coefficients and frictional pressure drops were examined and discussed. The results show that the local heat transfer coefficients of R1234ze(E) are averagely 33% and 18% lower than those of R600a and L-41b, respectively. The frictional pressure drops of R1234ze(E) are 21% lower than those of R600a but 6% greater than those of L-41b. Meanwhile, the experimental data of local heat transfer coefficients and frictional pressure drops are compared with some well-known correlations available in literatures.

Effect of lubricating oil on flow boiling characteristics of R-600a/oil inside a horizontal smooth tube

The aim of the present study is to experimentally investigate the effect of oil on flow boiling of R-600a as a hydrocarbon refrigerant inside a horizontal smooth tube. In order to study the effect of oil on the flow boiling of R-600a, a well instrumented apparatus was designed, fabricated and installed. The experimental conditions of this study include nominal oil concentrations from 0% to 2.5%, mass velocities in range of 130–380 kg/m2s, inlet vapor qualities from 0.05 to 0.77 and heat fluxes from 10 to 28 kW/m2which were conducted in a copper test tube with the inner diameter of 8.7 mm. Several parameters affecting the heat transfer coefficient and pressure drop of refrigerant–oil mixture, such as oil concentration, mass flux and vapor quality were investigated. The comparison between two-phase pressure drop for pure refrigerant and the refrigerant oil mixture reveals that pressure drop increases with the increase of oil concentration in all ranges of vapor quality and mass velocities. However, heat transfer coefficients tend to increase at low vapor qualities and decrease at the middle and high vapor qualities due to presence of oil.

Experimental investigation of thermal–rheological properties and heat transfer behavior of the heat transfer oil–copper oxide (HTO–CuO) nanofluid in smooth tubes

In this paper, thermophysical properties and convective heat transfer rate of the heat transfer oil–copper oxide nanofluid flow in horizontal smooth tubes is studied experimentally. At first, thermal–rheological characteristics of the heat transfer oil–copper oxide nanofluid, including density, volumetric thermal expansion coefficient, specific heat capacity, viscosity, and thermal conductivity, are measured at different temperatures. The heat transfer coefficient of HTO–CuO laminar flow under constant wall temperature boundary condition is also investigated. It is observed that the nanoparticle enhances the volumetric thermal expansion, viscosity, and thermal conductivity of the base fluid while decreases its specific heat. This behavior is also supported by some theoretical models. In addition, the Nusselt number is enhanced up to 32% using 1.5% CuO nanoparticles.

An experimental investigation and modelling of flow boiling heat transfer of isobutane-compressor oil solution in a horizontal smooth tube

Experimental results of local heat transfer coefficients for the boiling of working fluids (solutions of R600a with mineral naphthenic oil ISO VG 15) in a smooth tube with a small diameter (5.4 mm) are presented. The experiments have been performed in the following ranges: for the inlet pressure from 65.7 kPa to 82.2 kPa, for the heat flux from 2500 to 3300 W m−2, and for the mass velocity of the working fluid from 11.90 to 15.99 kg m−2 s−1). The quantitative estimation in reduction of the heat transfer coefficient of the wetted surface in the evaporator at a high oil concentration in the mixture is examined. The influence of heat flux and mass velocities on the values of the local heat transfer coefficients is analyzed. The equation for the modelling of the local heat transfer coefficient for boiling of an isobutane/compressor oil solution flow in the tube is suggested.

Numerical Study of Nanofluid Condensation Heat Transfer in a Square Microchannel

This study presents a numerical study of nanofluid condensation heat transfer inside a single horizontal smooth square tube. The numerical results are compared to previous experimental predictions, and show that the heat transfer coefficient can be improved 20% by increasing the volume fraction of Cu nanoparticles by 5% or increasing the mass flux from 80 to 110 kg/m2 s. Reducing the hydraulic diameter of the microchannel from 200 to 160 µm led to an increase in average condensation heat transfer coefficient of 10%. A new correlation estimating Nusselt number for condensation of nanofluids or pure vapor is proposed. It predicts average condensation heat transfer, with good agreement with the computed values.

Heat transfer characteristic of R-600a during flow boiling inside horizontal plain tube

In this paper, heat transfer characteristics and flow visualization of the hydrocarbon refrigerant R-600a during flow boiling inside horizontal smooth tube are investigated. For this purpose the well instrumented experimental apparatus including pump, flow meter, pre-heaters, test evaporator, by-pass and condenser was designed, fabricated and installed in order to measure the experimental data on flow boiling heat transfer and flow pattern of hydrocarbon refrigerant. The test section was a plain copper tube with an inside diameter of 8.7mm and a wall thickness of 0.41mm. The experimental tests were carried out varying: (i) the refrigerant mass fluxes within the range of 130–380kg/m2s; (ii) the vapor qualities up to 0.70; and (iii) the heat fluxes from 10 to 27kW/m2. The results indicate that, intermittent and annular flow patterns are the dominant flow patterns in the ranges of operating parameters of the present study. Also, the flow boiling heat transfer coefficient increased as the mass velocity increased in the fixed vapor quality. Moreover, heat transfer coefficient increased by the increase of vapor quality as the mass velocity was fixed. Most of the flow boiling heat transfer and flow pattern prediction methods have been developed for HFC and HCFC and CFC refrigerants and there are no general methods which can be applicable for all fluids. Therefore the present study experimental data for flow boiling heat transfer coefficient and flow pattern were compared with some existing models in order to find out which method could predict the convective boiling heat transfer and flow pattern characteristics of R-600a as a hydrocarbon refrigerant more accurately. The results indicate that Kattan–Thome–Favrat [23] flow map was closer to the flow patterns observed in this study. In addition, Gungor–Winterton [8] correlation had the best consistency with the present study experimental heat transfer data.

Experimental investigation during condensation of R-600a vapor over single horizontal integral-fin tubes

In this study, condensation heat transfer coefficients (HTCs) of refrigerant R-600a (iso-butane) over a horizontal smooth tube of outer diameter 19mm and five integral fin tubes of different fin-densities (945, 1024, 1102, 1181, and 1260fpm) were determined at vapor temperature of 39±0.5°C with different wall sub-cooling temperatures in a range of 3–12°C. Like other refrigerants, condensation heat transfer coefficients of R-600a showed the same trend with wall sub-cooling where condensation HTCs decrease with the rise in wall sub cooling temperatures. The heat flux was 11–20kW/m2 for the plain tube and 28–82kW/m2 for integral fin tubes. Based upon the data taken in this study, different graphs were plotted varying different parameters to show their dependency on other parameters. The experimental data were also validated by comparing them against different models.