Vibrations of the heat transfer devices can affect bubble formation and detachment processes, leading to instability at the liquid-vapor interface in convective boiling, thereby impacting heat transfer performance and stability in heat transfer systems. This paper investigates the influence mechanism of mechanical vibration on film boiling using the Volume-of-Fluid and Level Set (VOSET) approach. The impacts of vibration amplitude and frequency on film boiling heat transfer characteristics are comprehensively examined. Parameters such as bubble detachment volume (Vb), V b ), detachment time (td), t d ), and bubble rise velocity (ub) u b ) are analyzed to elucidate the influence mechanism of mechanical vibration on the film boiling heat transfer process. Results show that with increasing frequency, the bubble nucleation site and detachment frequency significantly increase, the wall temperature (Tw) T w ) decreases noticeably, and the Nusselt number (Nu) Nu ) increases. Compared to the non-vibration scenario, the average Nusselt number (Nuavg) Nu avg ) increases by 15.1 % at a vibration frequency of 10 Hz. Increasing the amplitude effectively promotes the film boiling heat transfer process, but the enhancement effect gradually diminishes. When the amplitude is 3 mm, Nu avg increases by 26.7 % compared to no vibration. The study further explores the impact of vibration on the instability of the vapor film. Vibration significantly alters the dynamic process of bubble growth and detachment. Increasing the vibration frequency and amplitude can enhance bubble rise speed, but with more complex periodic oscillations as frequency and amplitude increase. Additionally, vibration significantly shortens t d , further optimizing heat transfer efficiency. This study uncovers how vibration affects film boiling, offering crucial insights for designing and optimizing heat transfer enhancement techniques.

Pool boiling with high-efficient heat transfer process is crucial to chemical engineering, nuclear engineering, electronic cooling, and other energy conversion applications. Surface modification technology has attracted more attention in recent years as a simple and reliable boiling enhancement technique. Most of the studies using surface modification technology have focused on the enhancement effect of rectangular micro-structures, only a few researchers have adequately investigated the enhancement effects on pool boiling heat transfer and mechanisms of other complex micro-structures (like circle, trapezoid, or inverted trapezoid). In the present study, the pool boiling processes on surfaces with trapezoidal/inverted trapezoidal micro-pillars were numerically investigated using lattice Boltzmann method (LBM). The bubble dynamics and heat transfer characteristics during the pool boiling processes on surfaces with different pillar bottom angles (30(degrees) < theta < 135(degrees)) were compared under different wall-superheat conditions. The orthogonal tests were adopted on micro-pillars to optimize the geometric parameters, including the bottom angle of the micro-pillar, the bottom width of the micro-pillar, the number of the micro-pillar, and the height of the micro-pillar. A new CHF correlation on the mixed wettability surface with trapezoidal/inverted trapezoidal micro-pillars was proposed. It can be found that the main mechanism of nucleate boiling heat transfer on the micro-pillars is the evaporation of the microlayer, and the pillar bottom angle reveals a remarkable effect on the pool boiling performance on the mixed wettability surfaces. The surface of theta = 135(degrees) has the minimum wall-superheat at the onset of nucleate boiling (ONB). Within a wide range of wall-superheat conditions, the heat transfer efficiency of pool boiling firstly increases with the increasing of the bottom angle of the micro-pillar and subsequently decreases. The surface of theta = 60(degrees) has the highest critical heat flux (CHF). The surface of theta = 90(degrees) has the highest heat transfer efficiency in the film boiling process. The results of orthogonal tests suggested that the bottom angle of the micro-pillar is the most significant influential factor, and the optimal geometrical combinations were proposed. The new CHF correlation has acceptable accuracy in predicting the CHF on surfaces with trapezoidal/inverted trapezoidal pillars, which provides guidance for the design and optimization of micro-structured surface modification in various pool boiling applications.

Helically coiled tubes (HCTs) have been widely used in chemical industry and nuclear engineering because of its compact structure and excellent heat transfer performance. This paper designed and built an experimental platform to study the flow boiling heat transfer (FBHT) of subcritical water with high temperature and high pressure in an HCT. The results showed that the wall temperature on the inner side is higher and it gradually decreases when moving to the outer side along the circumferential direction. The variation law of circumferential wall temperature inhomogeneity (CWTI) under different pressures, mass velocities, heat fluxes and vapour qualities was obtained. The distribution of circumferential wall temperature varies greatly in the single-phase region while it varies slightly and is relatively uniform in the two-phase boiling region. As the vapour quality increases, the CWTI decreases first, and maintains at a low value when the vapour quality is about 0.0 ti 0.9. When the vapour quality reaches about 0.9, the CWTI increases rapidly. When the vapour quality is less than 0.5, the heat flux and mass velocity have little effect on the CWTI. When the vapour quality is larger than 0.5, reducing the heat flux or increasing the mass velocity can effectively reduce the CWTI. In addition, the flow pattern of two-phase boiling flow in the HCT is divided into four regions, namely bubble flow, slug flow, annular flow and mist flow, and the transition vapour quality xt1 = 0.2, xt2 = 0.5 and xt3 = 0.93 are respectively selected according to the sud-den abrupt change of the wall temperature or the differential pressure. Finally, the existing FBHT corre-lations are collected and evaluated and the calculation accuracy needs to be improved for more accurate calculation. Therefore, a new FBHT correlation with better accuracy is proposed for calculating the heat transfer coefficient of two-phase flow boiling in the HCT.(c) 2023 Elsevier Ltd. All rights reserved.

Advanced ultra-supercritical (A-USC) steam power generation technology is characterized by main steam conditions of > 700 degrees C and > 35 MPa, enhanced cycle efficiency, and reduced environmental footprint, and is one of the important development goals of the thermal power plants today. However, scarce studies were conducted on the heat transfer characteristics of supercritical water (SCW) under A-USC operating conditions. An experimental platform to safely measure the heat transfer of SCW flowing in a vertical tube at temperature up to 760 degrees C and pressure up to 42 MPa was designed and built in this study. Experimental measurements of the heat transfer characteristics of SCW were performed on this platform in a wide range of temperature and pressure, up to a maximum bulk fluid temperature of 750 degrees C and a maximum pressure of 38 MPa. The effects of pressure, mass flux, and heat flux on the heat transfer characteristics of SCW in the ultrahigh enthalpy region (over 3200 kJ center dot kg(-1)) were then analyzed based on the experimental data. Results show that in the ultrahigh enthalpy region, the bulk fluid temperature and wall temperature increase approximately linearly with increasing bulk fluid enthalpy. The heat transfer coefficient decreases gently and then gradually stabilizes in the same range, with values in the range of 4-10 kW center dot m(-2)center dot K-1. In the ultrahigh enthalpy region, increasing the mass flux or decreasing the heat flux will improve the heat transfer performance. And at a pressure of > 35 MPa, increasing the pressure will improve the heat transfer performance moderately. By comparing with experimental data, the Gupta's correlation has acceptable accuracy in predicting the heat transfer coefficient and inner wall temperature.

Analysis of the flow and heat transfer characteristics of supercritical water (SCW) become complicated by the extensive changes in the physical properties of SCW near its pseudo-critical line and by the secondary flow stemming from the buoyancy and centrifugal force in a helically coiled tube (HCT). To address the heat transfer deterioration (HTD) of SCW in HCTs, we investigated the distribution of circumferential wall temperatures and heat transfer coefficients of SCW in a vertical HCT under the HTD condition via numerical simulation. Wall temperature was highest on the inner side, and the local heat transfer coefficient was the lowest. With increasing heat flux, the wall temperature of each circumferential position around the tube rose, while the inner wall temperature rose the most. To explain the mechanism of HTD, the rapid decrease of the specific heat and thermal conductivity near the inner side played a certain role in deteriorating the heat transfer. Further, the buoyancy effect, and the thermal acceleration effect could be used to quantitatively assess the HTD condition. The threshold values of the dimensionless number Bu describing buoyancy effect and the dimensionless number Ac describing thermal acceleration effect were found to be 3.0 x 10-6 and 1.28 x 10-6, respectively. The concept of circumferential wall temperature inhomogeneity (CWTI) was used to quantify differences in local wall temperature around the HCT. CWTI significantly increased with increasing heat flux. At low heat flux levels (qw = 300-700 kW m- 2), the CWTI decreased at first, and then increases. When the heat transfer deteriorated with increasing heat flux, the trend of CWTI reversed. These results indicated that differences in circumferential wall temperature became larger under HTD conditions. Increasing pressure reduced the physical property changes in the HCT, leading to more uniform wall temperatures and a smaller CWTI. Under a constant ratio of heat flux to mass velocity, increasing the mass velocity weakens the heat transfer and even triggers HTD. In contrast, increasing pressure suppresses HTD.

Film boiling of magnetic nanofluid (MNF) in non-uniform magnetic fields generated by current carrying wires is numerically studied in this paper. The calculation model for the flow and heat transfer of MNF film boiling is established by incorporating both Maxwell's equation for magnetic fields and transport equations for interfaces into the basic governing equations of two-phase flows. The interface of boiling vapor film is captured by the VOSET method. Influence of the non-uniform magnetic field generated by single and multiple current carrying wires on the MNF film boiling is studied by analyzing the coupling among the magnetic field, flow field and interface distribution based on the numerical results. It is shown that the locations of the current carrying wire have a great influence on the MNF film boiling. The non-uniform magnetic field generated by current carrying wires may promote or inhibit the growth and departure of the vapor bubbles, which is determined by the distance between the current carrying wire and the top of initial vapor film. Compared with the cases without magnetic field, when the current carrying wire is near the bottom of the initial vapor film (located at x = 0, lambda(0)/6, 5 lambda(0)/6 and lambda(0)), the average wall temperature of the heated wall decreases and the average Nu number increases, resulting in enhancing the heat transfer. While the wire is below the bottom of the initial vapor film and H-0 = 10 kA m(-1), the averaged Nu number increases to 2.98, which is 4.2% higher than that in the case without magnetic fields. However, when the wire is near the top of the initial vapor film (located at x = lambda(0)/3, lambda(0)/2 and 2 lambda(0)/3), the bubble departure time increases slightly, and the averaged Nu number decreases by 1.1%. Further, the influence of the coupled magnetic field of four current carrying wires on the MNF boiling heat transfer is explored. Compared with the single wire, the coexistence of multiple current carrying wires can further promote the heat transfer performance of MNF film boiling. When the H-0 = 20 kA m(-1) generated by four current carrying wires, compared with the condition without magnetic field, the average Nu number increases from 2.87 to 3.80, increasing by 32.4%.

To explore the flow and heat transfer characteristics of supercritical fluid with high mass velocity in a helically coiled tube, experiments and numerical simulations were conducted to evaluate the wall tem-perature distribution and heat transfer coefficient in a vertical helically coiled tube under a pressure of 24-28 MPa, with a mass velocity of 250 0-40 0 0 kg middotm -2middots -1 and with a heat flux of 210-420 kW middotm -2. Results show that the coil's inner wall temperature is the highest and the outer wall temperature is the lowest because the centrifugal effect is much stronger than the buoyancy effect at a high mass velocity in a helically coiled tube and thus the fluid with lower temperature and consequently higher density shifts to the outer side of the coil. To describe the wall temperature difference in the different circum-ferential points, a metric called the circumferential wall temperature inhomogeneity (CWTI) is proposed, and the influence of the parameters on the CWTI is then analysed. As bulk fluid temperature increases, the CWTI first decreases to a minimum at the pseudo-critical temperature and then increases thereafter. The increase of mass velocity reduces the CWTI, and the influence of mass velocity on the CWTI is the most significant in the high enthalpy region. In the high enthalpy region with the bulk fluid tempera-ture of 410.1?, when the mass velocity increases from 2500 to 3250 kg middotm-2middots -1, the CWTI decreases by 34.2%. The CWTI also increases almost linearly with the increase in heat flux. Finally, the factors influenc-ing the average heat transfer coefficient in the helical tube are discussed. The coefficient first increases and then decreases with increase in the bulk fluid temperature, and the peak value of the coefficient appears at the pseudo-critical point corresponding to the supercritical pressure. Increasing the mass ve-locity improves the average heat transfer coefficient, and the improvement effect is more obvious after the pseudo-critical point. Increasing the pressure causes the bulk fluid temperature at which the peak average heat transfer coefficient is achieved to increase, and the peak value gradually decreases. When the pressure increases from 24.0 MPa to 26.0 MPa and to 28.0 MPa, the peak value of the average heat transfer coefficient decreases by 34.2% and 46.4%, respectively.(c) 2022 Elsevier Ltd. All rights reserved.

The Allam cycle is a promising oxy-fuel combustion power cycle with high electrical efficiency and near-zero carbon dioxide emissions. In this paper, the thermodynamic and exergoeconomic analyses are performed for a novel combined power and cooling oxy-fuel power cycle, which combines the Allam cycle with liquefied natural gas regasification process. Parametric study is conducted to investigate the effects of key cycle variables on the electrical and exergy efficiencies and total product unit cost of the proposed cycle. Multi-objective optimization is carried out to maximize the exergy efficiency and minimize the total product unit cost. The results show that Condenser 2 has the highest exergy destruction of 22.81 MW, followed by the combustor (22.72 MW). The combustor, Condenser 2 and gas turbine are the three most important components from exergoeconomic aspects. The introduction of adiabatic compression heat of the bypass stream has a positive impact on the system performance, especially when the outlet temperature of the combustor is low. The optimization results indicate that the exergy efficiency and the total product unit cost cannot reach the optimal values at the same time. The highest exergy efficiency of 50.31% and the lowest total product unit cost of 16.654 $/GJ are obtained respectively with different sets of cycle variables. In addition, the electrical efficiency of the optimized proposed cycle is up to around 65.7%, about 11 percentage points higher than that of the Allam cycle.

The horizontal tube falling film evaporation technology is one of the most promising desalination methods owing to its various advantages. The pressure drop of vapor plays a pronounced role in evaporation efficiency in low-temperature multiple-effect desalination (LT-MED). In this study, an experimental test bench by using air in the horizontal direction across a tube bundle with a vertical downward falling film was designed, and the flow characteristics of vapor of a large-scale desalination plant were simulated. Falling film pressure drops (FFPDs) under different conditions of air-inlet velocity, water-spray density, heating flux, and tube column number were obtained and analyzed. The experimental results indicate that FFPD can quadratically increase with the rise in air velocity, linearly rise with increments of spray density, and rapidly grow with the cumulative increase in tube columns. However, heat flux presented a minimal effect on FFPD because of the predominant portion volume of airflow relative to that of evaporated vapor. On the basis of the dimensionless analysis and the least-square multiple regression, a brand-new fitting correlation was proposed to predict FFPD. The proposed method corresponded well with the experimental data within the deviation of +/- 15%. The correlation also estimated the inter-tube average vapor velocity to be within 5-20 m/s in the first-effect evaporator of field-scale LT-MED in engineering. The brand-new correlation can also provide a theoretical reference in engineering design that considers vapor velocity and flow resistance in LT-MED.

Surface modification technology by controlling the surface wettability or applying micro/nano-structures to enhance the boiling heat transfer performance has attracted a great deal of interest in recent years. Abundant experiments were performed to investigate the boiling processes on the modified surfaces, and lots of experimental data as well as reliable conclusions were obtained. With the advantages of saving costs and time, the numerical method has been a new reliable way to investigate the bubble dynamics and heat transfer during pool boiling processes on the modified heated surfaces. Pseudopotential LB model is capable of simulating the entire boiling processes including the bubble nucleation, and this model has been successfully applied to simulate the pool boiling processes on both the smooth surfaces and the hydrophilic-hydrophobic mixed surfaces with micro-pillars. However, the numerical simulations of the boiling processes on the mixed surfaces were still rare, and the influence of geometrical parameters of pillar structures of the mixed surface on boiling heat transfer performance was still unclear yet. Moreover, some of the conclusions in existing literature were inconsistent with those in others. Thus it's necessary to carry out more numerical and experimental researches to solve these problems. In this study, the MRT pseudopotential model coupled with phase-change model was applied to simulate the pool boiling processes on the hydrophilic-hydrophobic mixed surface textured with micro-pillars. Under different wall superheats, the bubble dynamics and heat transfer during boiling processes on the mixed surface with micro-pillars were compared to those on the smooth hydrophilic surface and the hydrophilic-hydrophobic mixed surface without micro-pillars. The heat transfer enhancement mechanism of the mixed surfaces was revealed and the influence of geometrical parameters of pillars, including pillar width and pillar number, on bubble dynamics and heat transfer performance during pool boiling processes was investigated in detail.

3D numerical simulation on the steam velocity field and flow resistance configuration in the fullscale low-temperature multi-effect desalination (LT-MED) evaporator were conducted and analyzed.The complicated two-phase water-vapor flow through the tube bundle was modeled using the single-phase vapor flow through the porous media (PM) model, which could simplify the effect of anfractuous geometry of the flow path and interactions between the two phases in the tube bundle.A PM model has been validated by additional experimental results.3D numerical results were justified between the literature and real LT-MED plants.The numerical results suggest that the steam velocity in the tube bundle ranges from 1 to 12 m/s and presents different variation trends along the direction of the tube row, tube column, and tube length.In the axial channel, steam velocity first exhibits a rising tendency up to maximum of 50 m/s close to the outlet and then greatly decreases beyond the outlet.The components of steam flow resistance comprise tube bundle, demister, steam channel, and tube-side condensation resistances.The largest flow resistance is contributed by the axial steam channel, accounting for 57.6% proportion.The small steam flow resistance with a total of 381.6 Pa in the first-effect evaporator causes a significant reduction in the effective heat transfer temperature difference by a proportion of 12.2%.This study endeavors to find a new possibility of numerical simulation for the entire LT-MED evaporator, which could provide a good reference to engineering-optimized design and modeling of the LT-MED running condition.

Wall temperature of heat transfer tubes is one of the most important parameters indicating the operation safety of various heat transfer facilities, and as a result, estimation of the wall temperature becomes one of the main tasks in the design of heat transfer facilities. The wall temperature look-up table (Tw-LUT) can be established directly from experimental data and can be then used to estimate the wall temperature of the heat transfer tubes, avoiding the approximation or extrapolation of fluid properties that inevitably exists in the heat transfer correlations. In view of the problems existing in applications with wall temperature estimations of the heat transfer tube, such as limited data points and the limited application scopes of parameters, a look-up table is built in this paper for wall temperatures of vertically-upward round tubes of 10 mm tube diameter with heat transfer to supercritical water (SCW), under conditions with pressure in the range from 22.5 to 31 MPa, the mass velocity in the range from 200 to 3000 kg.m(-2).s(-1), the heat flux in the range from 200 to 1800 kW.m(-2), and the bulk fluid enthalpy in the range from 1000 to 3000 kJ.kg(-1). In order to cover the gaps between the experimental data points, and to improve the prediction accuracy of the Tw-LUT, the best heat transfer correlation is selected for each local area of interest in the LUT based on its prediction accuracy in the corresponding local area, and then the best heat transfer correlation is adopted to supplement wall temperature results to fill up the Tw-LUT. The comparison between the wall temperatures by the Tw-LUT and the experimental wall temperatures is carried out to verify the accuracy of the Tw-LUT, and it is shown that the mean absolute deviation of the results is 0.87%, and 87.81% of the results fall into the +/- 3% error band, indicating that the Tw-LUT has a good accuracy for wall temperature prediction and the establishment method is reliable and can be used to build other look-up tables. The Tw-LUT can be applied not only to normal heat transfer conditions but also to deteriorated heat transfer conditions and enhanced heat transfer conditions with a satisfactory accuracy.

To overcome the problem of abnormally large bubbles and the large reduction of heat flux under low gravity, the computational model of magnetic nanofluid (MNF) boiling flow was used to systematically study the thermodynamic characteristics of an MNF-saturated film boiling with and without the magnetic field. This study found that in the absence of a magnetic field, the decrease of the gravity level makes the bubble size increase and the bubble departure time increase, and the lower the gravity level, the worse the boiling heat transfer. However, after applying the magnetic field, bubble size decreases significantly and the bubble departure time is shortened. As the magnetic field intensity increases, the difference in bubble size and heat transfer characteristics between different gravity levels becomes smaller and smaller, which shows that for the boiling flow of MNF under low gravity levels, applying a magnetic field can effectively avoid the appearance of abnormally large bubbles, enhance heat transfer, and improve the safety of related heat transfer equipment.

The Allam cycle is one of the most promising oxy-combustion systems and has attracted much attention from the energy industry. A novel layout of the Allam cycle, which combines a reheating configuration with the original Allam cycle, was proposed in the present study. The effects of key variables on the net cycle efficiency and net specific work of the new system were investigated. The Allam cycle with reheat was optimized to maximize the cycle efficiency and compared with other cycles. The results showed that the minimum cycle temperature and the outlet temperature of the second combustor have significant effects on the cycle performance, while the maximum cycle pressure has minimum effect on the cycle performance. The highest exergy destruction of the optimized cycle is contributed from the two combustors, which accounts for 25.42% of the total input exergy. The net efficiency of the optimized Allam cycle with reheat is up to 49.32%, a little higher than those of other oxy-combustion cycles and about 5 percentage points lower than that of the original Allam cycle. However, the net power output and net specific work are up to 904.6 MWe and 606.2 kJ/kg for this novel system, about 2.1 times and 2.2 times of those of the original Allam cycle, respectively.

Gas-liquid two-phase flow systems are widely existed in many engineering systems and industrial processes, and measurement techniques for two-phase flows are crucial in a diverse range of engineering applications. Most of the existing measurement techniques usually brought extra resistance to the flow in the branch pipes and, to some extent, influenced the original distribution of the gas-liquid two-phase flows in the parallel T-junction system. In order to study the distribution characteristics of two-phase flow in vertically parallel T-junctions, the electromagnetic-conductance measurement method was developed to measure the void fraction and liquid flow rate of the gas-liquid two-phase flow in the branch pipes. Electromagnetic-conductance measurement technique has the advantages of high response rate, satisfactory accuracy and low disturbance on the flow field of gas-liquid two-phase flows. In order to calibrate the conductance measurement method, experiments were carried out with the liquid velocity range of 0.2–2 ​m·s-1, the gas velocity range of 0–50 ​m·s-1 and the volume void fraction range of 0–0.99. Through the calibration experiments, the relationship between the resistance value of each conductivity sensor and the volume void fraction of two-phase fluid was decided to be established as a logistic function. By the error analysis of the calibration results, 75.9% of experimental results of the void fraction in the gas-liquid two-phase flow fell within the ±20% error bands. The electromagnetic-conductance measurement method proposed in this paper could provide a reference for accurate and non-interference measurement of two-phase flow in parallel T-junctions.