Effect Of Mass Flux Research Articles

A computationally derived heat transfer correlation for in-tube cooling turbulent supercritical CO2

This paper computationally investigates the turbulent heat transfer of sCO2 flows cooled in large horizontal tubes with diameter of 15.75 mm, 20 mm and 24.36 mm using RANS turbulence models. The numerical models were validated against experimental data published in literature to demonstrate the reliability of CFD simulations on the heat transfer coefficient prediction and buoyancy effect capture to turbulent sCO2. Based on the validated model, a number of computations, involving a wide range of operating conditions, have been carried out. The effect of mass flux (200–800 kg/m2s), pressure (8–10 MPa), heat flux (5–36 kW/m2) and tube diameter has been analysed. Results demonstrate that the AKN model shows the best consistencies with the experimental measurements and is also able to well reproduce the heat transfer characteristics under various conditions. As the mass flux increases, the heat transfer coefficients go up due to the enhanced turbulence diffusion. Pressure has a significant effect on the distribution of heat transfer coefficient, and its peak drops sharply with rising pressure. At Tb > Tpc, with the heat flux and tube diameter increasing, sCO2 heat transfer performance is improved; whereas at Tb < Tpc, the heat flux and tube diameter almost have no effects on the heat transfer performance. Considerable deviations with the existing heat transfer correlations necessitate the development of a new correlation to predict the heat transfer coefficients of cooling turbulent sCO2 in large horizontal pipes. Based on the reliable computational datasets, a Nusselt number equation based on the Gnielinski form with the ratio of density incorporated is formulated.

Energetic transition within thermal machines and co-generation: effect of mass flux on critical heat flux

Topics associated with the critical heat flux (CHF) in industry sector are at the heart of the hot interesting issues on the global agenda. In this paper, we introduce a new method of energy analysis of heat transfer in new generation heat exchangers applicable to the innovative sector of energy recovery. A statistical analysis is performed to study the effect of mass flux on the critical heat flux of an evaporator boiler. The aim of the present study is to provide insights into the effect of mass flux on the CHF when porous tubes are used. Independent sets of full 23 factorial designs with three central points were employed in case of porous coated tubes. Results revealed that the CHF could be influenced by some factors which were dominated in the studied conditions. More specifications of CHF and its relation with the input variables were detailed in this paper.

Performance Evaluation of Helical Coil in Condensation Heat Transfer

This research work presents experimental investigation of condensation heat transfer characteristic of steam flowing inside the helical coil with cooling water flowing in shell in counter direction. The experiments were performed using steam saturation temperature ranging from 103 – 115 Degree Celsius and pressure 0.2 – 1 bar gauge. The experiments were performed with cooling water flow rate 3 l/min and 8 l/min. The effect of mass flux, heat flux and heat transfer coefficient for three different curvature ratio of stainless steel helical coil have been investigated. It is concluded that the experimental results of the helical coil are more superior to straight tube.

Heat transfer investigation of supercritical R134a for trans-critical organic Rankine cycle system

In order to restrict the adverse effects caused by the heat transfer deterioration of the supercritical fluid and to build a cost-effective Trans-critical Organic Ranke Cycle (TORC) system, the heat transfer of supercritical R134a was experimentally investigated in a horizontal ribbed tube for the first time. Knowledge that are important for the design of the supercritical heater and TORC system were explored in detail, including the general features of the heat transfer and effects of heat fluxes, pressures, and mass fluxes. It was found that large heat flux will decrease Nu in most cases, and the effects of pressure differed largely in different working conditions. The working conditions with low heat flux, large mass flue and pressure close to the critical pressure were recommended for the design of the supercritical heater and TORC system. Compared with vertical smooth tubes, the threshold for heat transfer deterioration criterion, Bo, was enlarged at least 7.5 times and the heat transfer was improved around 1.6 times among all the experimental data. Then, different types of previous correlations were evaluated and none of them showed a satisfactory accuracy. Finally, new correlations with better prediction accuracy were built to provide references for the TORC system design.

An improved DNB-type critical heat flux prediction model for flow boiling at subcritical pressures in vertical rifled tube

Based on the near-wall bubble coalescence model and the structural characteristics of the rifled tube, improvements and optimization of the model for rifled tube were conducted. DNB-type CHF phenomenon in the rifled tube under subcritical and near-critical pressures was investigated by numerical calculation. A new CHF prediction model of flow boiling in rifled tube with low qualities was established and compiled by FORTRAN language. The model used the Staub model which considering the influence of the bubble contact angle to calculate the bubble detachment diameter. The surface friction coefficient of the rifled tube was calculated by the classical Kohler and Kastner formula. And a new critical void fraction of the wall bubbly layer αb correlation was proposed based on a large number of CHF experimental data. The present model was verified by the CHF look-up values and CHF experimental values. Based on the CHF experimental data of the rifled tube, three classic CHF prediction models and the present CHF model were evaluated, and it was found that the present modified CHF model has the highest prediction accuracy for the rifled tube, which is the best agreement with the experimental values, indicating that the present model is appropriate for DNB-type CHF prediction of the rifled tube. In this paper, the effects of pressure, mass flux and inlet fluid subcooled enthalpy on CHF were studied. It was found that, the effect of pressure and mass flux on CHF is obvious in the low quality region, with the increase of vapor quality, the influence of pressure and mass flux on CHF weakens.

Simulation of cavitation enhanced temperature elevation in a soft tissue during high-intensity focused ultrasound thermal therapy

The present study aims to investigate temperature distribution caused by bubble oscillations in a soft tissue during focused ultrasound therapy by introducing a coupled temperature-cavitation model. The proposed model is capable of describing bubble dynamics, viscoelastic properties of surrounding tissue-like medium, temperature distribution inside and outside the bubble, vapor diffusion within the bubble and vapor flux through the bubble wall to the exterior. The continuous temperature distribution inside and outside the oscillating bubble in soft tissue subject to ultrasound wave with high acoustic pressure is presented. The temperature close to the bubble wall can reach the value of about 103 K. The elasticity of soft tissue reduces temperature values. The relaxation time effect strongly depends on the period of the ultrasound wave. If the vapor mass flux effect is taken into account in the simulations, the rectified growth effect can be observed, which can lead to the decrease of the temperature values. Due to the growth of the bubble, the effects of elasticity and relaxation time on the temperature become less prominent during several bubble oscillation cycles. The impact of cavitation heat source terms on the exterior temperature was examined and led us to draw conclusion that, even though these heat sources can increase the outside temperature values, they can not be treated as main mechanisms for the temperature elevation during a few microseconds. The performed comparison with uncoupled conventional model for the outside temperature calculation revealed that coupling with inside temperature model delivers incomparably higher values to the bubble’s exterior and, therefore, it is essential for the accurate description of the treatment process.

초임계 조건 내 CO₂-CH₄와 CO₂-N₂ 혼합물의 관 내측 열전달 계수의 수치적 해석

In this study, the effect of mixture mole fraction, mass flux, and operational pressure on the heat transfer coefficient in supercritical conditions was investigated using commercial CFD program of FLUENT for CCS (Carbon Capture & Storage) transporting process. Simulations were carried out with mole fractions of the impurities of CH₄ and N₂ at 1, 3, and 5%, the pressure at 80, 90, and 100 bar, and the mass flux at 200, 400, and 600 kg/m². The range of inlet temperatures were set to 10 K at 5 K interval based on the point where the maximum heat transfer coefficient was observed. For CO₂, the maximum heat transfer coefficient was found at 308 K, the value was 5,270 W/m²․K. When the mole fraction of CH₄ was 3%, it was 4,400 W/m²․K at 304.5 K. When mole fraction of N2 was 3%, it was 4,000 W/m²․K at 303 K. It was observed that as the mole fraction increases, the maximum heat transfer coefficient decrease, and the temperature at which it is maximum increases. It was confirmed that the type of impurities affect the heat transfer of the mixture. As the mass flux of mixture increase, the average heat transfer coefficient increases too.

Boiling Heat Transfer Characteristics for Methane, Ethane and Their Binary Mixtures

Methane (R50) and ethane (R170) are the dominated components of natural gas and the important components in mixture refrigerants for the mixture Joule–Thomson refrigeration cycle. In this article, experimental investigations on nucleate pool boiling and flow boiling heat transfer characteristics of R50, R170, and their binary mixtures are presented. The effects of saturation pressure, heat flux, mass flux, concentration, and vapor quality on heat transfer coefficients are analyzed and discussed. Firstly, the pool boiling heat transfer data were compared with six well-known correlations. Labuntsov correlation shows the best agreement with a mean absolute relative deviation (MARD) of 11.3%. Secondly, a new flow boiling heat transfer correlation for pure fluids was proposed based on the asymptotic addition of forced convection and pool boiling. The modified enhancement factor and suppression factor were developed to account for their relative contribution. In addition, in order to consider the mass transfer resistance of mixtures, a new mixture factor was deduced. The new flow boiling heat transfer correlations can well predict the experimental data with the MARD of 9.5% for pure fluids and 8.3% for mixtures.

Condensation heat transfer of R410A on outside of horizontal smooth and three-dimensional enhanced tubes

Experimental investigations of flow condensation on the outside surface of one smooth tube and two enhanced surface tubes with the same outer diameter of 12.7 mm were conducted. The refrigerant flowed in an annular space between the test tube and the outer tube. The newly developed 2EHT three-dimensional surfaces have multiple enhancement patterns made up of staggered petal arrays and parallel wavy channels. Tests were performed using refrigerant R410A over a mass flux range of 75–250 kg m−2 s−1, at average saturation temperatures ranging from 35 to 45 °C. For the smooth tube, the external heat transfer coefficient decreases first and then flattens out gradually at low mass fluxes. Relative significance of inertia force might be the major reason. New prediction correlations for smooth tubes based on the Nusselt's theory were presented, having all data points within ± 4% error band. The effects of saturation temperature, mass flux and vapor quality on heat transfer coefficients for all the tested tubes were analyzed. In addition, the condensation heat transfer coefficient ratio of the 2EHT-2 tube to a smooth tube is only 0.86–0.94 for mass fluxes ranging from 100 to 250 kg m−2 s−1. This phenomenon can be explained by the retention of condensate and surface tension effects. New correlations for film condensation in the annular channel outside the 2EHT tubes were also developed, showing a good agreement with experimental data.

Experimental Study of Condensation Heat Transfer and Pressure Drop inside a Small Diameter Microfin and Smooth Tube at Low Mass Flux Condition

This study was made to investigate the condensation local heat transfer and adiabatic pressure drop of R134a inside a 2.5 mm outside diameter smooth and microfin tube at low mass flux condition. Data were measured for mass fluxes from 50 to 300 kg m−2s−1, vapor qualities from 0 to 1 and saturation temperatures from 20 to 30 °C. The effects of mass flux, vapor quality, saturation temperature, microfin and diameter of test tube were analyzed. The pressure drop of the microfin tube was about 1.5 times higher than that of the smooth tube at mass flux 50 kg m−2s−1. The heat transfer coefficient of the microfin tube was about 2–5 times higher than that of the smooth tube at mass flux 100 kg m−2s−1. Experimental results were also compared with typical correlations. For the microfin tube, however, all the existing correlations do not adequately predict the present data. Poor predictions may be attributed to the lack of the small diameter microfin tube with low mass flux data in their database. Hence, it is essential to develop a condensation and pressure drop correlation for the small diameter microfin tube at low mass flux condition.

Effects of Heat and Mass Flux on MHD Free Convection Flow Through a Porous Medium with Radiation and First Order Chemical Reaction

The study of a magneto hydrodynamic (MHD) free convection flow of an incompressible viscous fluid flow past a vertical surface is analyzed by taking into account viscous dissipation under the influence of radiation effect and chemical reaction with constant heat and mass fluxes. The governing partial differential equations have been converted into a set of ordinary differential equations using non dimensional quantities. The perturbation technique has been applied to solve the system of partial differential equations. The velocity, temperature and concentration fields have been studied for the effect of the permeability parameter (α), Grashof number for heat transfer (Gr), Grashof number for mass transfer (Gm), Prandtl number (Pr), magnetic parameter (&lt;i&gt;M&lt;/i&gt;), chemical reaction parameter (&lt;i&gt;Kr&lt;/i&gt;), Schmidt number (Sc), Eckert number (E), radiation parameter (&lt;i&gt;F&lt;/i&gt;) etc. The expressions for the skin-friction, rate of heat transfer and rate of mass transfer are also derived. It is observed that when the radiation parameter increases, the velocity increases near the vertical surface. It is also seen that the concentration decreases with an increase in the chemical reaction parameter (&lt;i&gt;Kr&lt;/i&gt;).

Experimental study of critical heat flux in flow boiling under subatmospheric pressure in a vertical square channel

Critical Heat Flux (CHF) is the maximal limit of heat flux in two-phase nucleate boiling heat transfer; therefore, an understanding of CHF under a wide range of conditions is important for safe system operation. In this work, CHF experiments are conducted over a range of subatmospheric system pressures in a vertical square channel that is heated on one side. The experimental conditions cover a pressure range of 20 kPa to 108 kPa, a mass flux range of 45–190 kg/m2-s, and an inlet subcooling range of 0–14 K. Heat flux is gradually increased until an excursion of the wall temperature occurs, indicating CHF. For the experimental conditions considered, CHF increases with rising system pressure, mass flux, and inlet subcooling, although the effects of mass flux and inlet subcooling are weak. A new correlation for CHF is developed and found to predict the data with an average error of ±15.6%.

Investigation of the CHF correlation for a narrow rectangular channel under a downward flow condition

In present study, CHF experiments were conducted to evaluate and improve existing CHF correlations for narrow rectangular channels under a downward flow condition. A test section heated on both sides simulates a subchannel of plate-type fuels in a research reactor. The experiment covers a wide range of flow conditions: 0–2739 kg/m2 s for mass flux, 37–99 °C for inlet temperatures and 120–224 kPa for outlet pressures. The experimental data obtained in the present study are compared with prediction results of existing CHF correlations. From the evaluation, the correlation proposed by Kaminaga et al., (hereafter referred to as Kaminaga correlation) shows the best prediction performance for the experimental data among them. However, it was also found that the Kaminaga correlation still contains a large degree of uncertainty relative to the experimental data. In this study, we attempted to improve the Kaminaga correlation to minimize its prediction uncertainty. For this purpose, we investigated the gap size, mass flux and subcooling effects on the CHF in a narrow rectangular channel based on both the present and Mirshak CHF data. Finally, the Kaminaga correlation was modified to consider the effects of the gap size, mass flux, and subcooling on the CHF.

A REVIEW OF SINGLE-PHASE AND TWO-PHASE PRESSURE DROP CHARACTERISTICS AND FLOW BOILING INSTABILITIES IN MICROCHANNELS

This study presents a comprehensive review on single and two-phase pressure drop characteristics and flow boiling instabilities in microchannels to outline the discrepancies in the literature. The effect of mass flux, heat flux, experimental conditions, channel geometrical parameters (hydraulic diameter, aspect ratio etc.) was surveyed critically over a broad range of literature studies including the past and recent researches. Additionally, conventional and micro-scale pressure drop correlations are discussed. This study showed that the continuum theory is applicable for single-phase pressure drop applications with some considerations. Also, the two-phase pressure drop in micro-scale was found to have similar characteristics with conventional-scale channels. On the other hand, flow boiling instability is one of the important issues in microchannel heat exchangers. Flow boiling instabilities affect the system performance and may cause system failures. Therefore, unstable flow boiling conditions were identified in this paper for a reliable design of micro heat exchangers.&amp;nbsp;

Effect of hydraulic diameter on flow boiling in rectangular microchannels

An experimental study is conducted to investigate the effect of hydraulic diameter on the saturated flow boiling characteristics of deionized water in parallel rectangular microchannels. Experiments have been performed for the mass fluxes of 51, 65, 78 and 93 kg m−2 s−1, and the hydraulic diameters of 100, 150, 200 and 250 μm. The wall heat flux ranges from 35.9 to 105.6 kW m−2. To eliminate the effect of aspect ratio and to address clearly the effect of hydraulic diameter, all the channels are designed with square cross section. Flow visualization studies are performed for a better understanding of the underlying physical phenomena. Effects of heat flux, mass flux and vapor quality on the heat transfer and total pressure drop have been investigated, too. It is concluded that hydraulic diameter has significant influence on both of the local two phase heat transfer coefficient and the total pressure drop.

Two-phase simulation of the pressure loss in helical channel

The CFD simulations are carried out for the flows in a helical gas-liquid separator, which is installed at the inlet of the electric submersible pump (ESP) to separate gas from gas-liquid mixture and keep the efficiency of the pump. The effects of mass flux, curvature and helix angle on two-phase pressure drop have been investigated. Through theoretical analysis and numerical simulation, it is found that the mass flux of two-phase flow and the curvature of spiral passage have influence on pressure drop, while the spiral angle has little effect on pressure loss.

Modeling of flow boiling heat transfer coefficient of R11 in mini-channels using support vector machines and its comparative analysis with the existing correlations

In recent years, extensive research efforts have been devoted to flow boiling heat transfer mechanisms in macro and mini-channels. However, it is still difficult to predict the flow boiling heat transfer coefficient with satisfactory accuracy. In this study, support vector regression (SVR) models have been constructed using a respectable experimental database (767 samples) from the literature to predict the heat transfer coefficient of R11 in mini-channels for subcooled (324 samples) and saturated (443 samples) boiling regions. The prediction performance of the SVR-based models have been evaluated based on the statistical parameters. SVR-based models have been found to exhibit an average absolute relative error (AARE) of 1.7% and correlation coefficient (R) of 0.9996 for subcooled boiling, while for saturated boiling the values of AARE and R are 1.6% and 0.9993, respectively. Also, the developed SVR-based models have been compared with the well-known existing correlations. The superior prediction performance of SVR-based models has been observed with the lowest value of AARE and the highest value of correlation coefficient (R). Furthermore, parametric effects of mass flux, vapor quality, heat flux and pressure on the flow boiling heat transfer coefficient have also been investigated and the SVR-based models have been found to agree well with the experimental results.

Heat transfer measurement and flow regime visualization of two-phase pulsating flow in an evaporator

Heat transfer and flow regime of two-phase R134a pulsating flow in an evaporator have been studied experimentally in this work. Heat transfer coefficient was measured and liquid-vapor two-phase flow regimes was observed under different mass flux, inlet vapor quality and pulsating period. Results show that heat transfer can be enhanced by pulsating flow in short periods (<16 s) with a maximum enhancement of 28% compared with the continuous flow; a deterioration of heat transfer is also observed for flow under long pulsating periods for some conditions, which could be as much as 30%. The effect of mass flux and inlet vapor quality is complex and coupled with pulsation period, which might result from the difference of flow developing between different pulsating periods. The instantaneous flow regime is recorded by a high speed camera and statistically analyzed to explore the mechanism of pulsating flow on heat transfer performance. The statistical analysis of flow regime from the recorded video shows that the flow regime of pulsating flow at on-time remains the same as that of continuous flow with the same instant mass flux, and the flow regime at off-time is mostly stratified, stratified wavy flow depending on the pulsating period. The transition of stratified-wavy flow regime to intermittent, annular flow regime of pulsating flow is the main contribution to the enhancement of heat transfer while dry-out during off-time plays an important role in heat transfer deterioration.

Experimental study on single-phase frictional pressure drop for water flow under high heat fluxes

Pressure drop is one basic issue for the heat-removal systems for high heat flux components, such as the ITER divertors in fusion reactors, in which an accurate knowledge of single-phase frictional pressure drop is fundamental for further calculation of two-phase frictional pressure drop. However, the single-phase frictional pressure drop is still not well examined under high heat fluxes. In this study, experiments were conducted on the single-phase frictional pressure drop of water flow in circular tubes within the range of q = 5–10 MW/m2, G = 6000–10,000 kg/m2 s and p = 3–5 MPa (typical operating parameters for water-cooled divertors). The data were correlated using the equation of f/fad =(μw/μb)m, in which m is an empirical exponent. It is found that the exponent m is affected by the length-to-diameter ratio of flow channel, while the effects of mass flux and heat flux are insignificant. In addition, a number of existing correlations for single-phase friction factor were compared with our data. The results indicate that these correlations cannot well capture part of the data. Therefore, a modified correlation is proposed to calculate the single-phase friction factor under high heat fluxes, in which the effect of length-to-diameter ratio is taken into account. The new correlation has an improved performance compared with the existing correlations.

Supercritical heat transfer characteristics of R1233zd(E) in vertically upward flow

The recently developed low-global-warming-potential working fluid R1233zd(E) is regarded as an alternative to R245fa in the organic Rankine cycle (ORC) considering the environmental impacts. In this study, the heat transfer characteristics of R1233zd(E) at supercritical pressures were investigated experimentally in a vertically upward flow inside a tube. The inner diameter and length of the test tube were 4 mm and 1.04 m, respectively. With the known supercritical heat transfer characteristics of R245fa, the experiment was conducted under the mass fluxes of 400 and 600 kg/m2 s, heat flux from 20 to 80 kW/m2, and pressure of 3.93 and 4.40 MPa, which were equal to the reduced pressure (P/Pcri) of 1.10 and 1.23, respectively. The effects of mass flux, heat flux, and pressure are discussed based on the experimental data. The experimental results are compared with existing correlations and the correlation proposed by Petukhov et al. shows the best predictability. Moreover, a comparison with the heat transfer coefficients of R245fa is presented. For the new R1233zd(E) working fluid, the abrupt deterioration characteristics did not occur under the experimental conditions, and a higher heat transfer coefficient was obtained in the supercritical region, in comparison to that obtained with R245fa.