Heat Transfer Of Supercritical Water Research Articles

Investigation on heat transfer of in-tube supercritical water cooling accompanying out-tube pool boiling

Heat transfer of in-tube supercritical fluid cooling accompanying out-tube pool boiling was investigated. A smooth horizontal circular tube with an inside-diameter of 20 mm was submerged in a water pool at atmospheric pressure. Test parameters of in-tube were as follows: Pressure: 23–28 MPa, mass flux: 600–1000 kg·m−2·s−1, fluid temperature: 400–725 K, and the temperature difference between bulk and wall: 300–374 K. A thermal amplification system based on out-tube pool boiling was used to improve the measurement accuracy of local heat duty near pseudocritical region. According to the experiment, the transition from nucleate boiling to film boiling in the pool occurred near the pseudo-critical fluid region. Sharp variation on thermo-physical properties led to the peak value of heat transfer coefficient in the pseudo-critical region. The pool boiling heat flux increased gradually to 1.19 MW·m−2 near the pseudocritical point. Based on the experimental data, a modified Gnielinski equation was adopted to predict the heat transfer coefficient of in-tube supercritical fluid cooling with out-tube pool boiling. The thermophysical property ratio of the wall to the bulk as well as the effect of buoyancy were taken into consideration in the new correlation. The predicted correlation has an error of less than 20% to the experimental data.

Analysis of heat transfer of supercritical water by direct numerical simulation of heated upward pipe flows

Direct numerical simulation (DNS) was performed to investigate the turbulent heat transfer of supercritical water in an upward pipe with and without buoyancy. The heat transfer deterioration mechanisms were comprehensively studied by analyzing the effects of large thermophysical property variations, flow acceleration and buoyancy. It is shown that the large property variations near the wall severely impair the heat transfer in supercritical water especially for forced convection. The flow acceleration induced by thermal expansion shows an obvious suppression on turbulence but does not affect the heat transfer significantly due to the quick developing of thermal boundary layer. A small buoyancy first impairs the heat transfer and then improves the heat transfer due to the turbulence attenuation and recovery. With buoyancy increasing, the turbulence recovery gets earlier and stronger, and the wall temperature is significantly reduced due to the large buoyancy production of turbulent kinetic energy (TKE) in supercritical water flows. The instantaneous vortical structures based on Q criterion also clearly show the process of turbulence attenuation and recovery. In addition, the simulation using two eddy-viscosity turbulent models is compared to DNS data. The results show that the failed prediction of wall temperature and Nusselt number by those turbulent models is possibly due to the bad results of TKE, production of TKE and turbulent Prandtl number (Prt).

Experimental study of wall-to-bed heat transfer in a supercritical water fluidized bed

Supercritical water (SCW) fluidized bed is a new reactor concept for hydrogen production from biomass or coal gasification. This paper presents an experimental study of wall-to-bed heat transfer characteristics for Geldart-B particles in SCW fluidized bed with the conditions of pressure ranges from 22.3 MPa to 25.7 MPa and temperature ranges from 651 K to 730 K. The effects of velocity, temperature and pressure on wall-to-bed heat transfer coefficient in SCW fluidized bed were discussed. The wall-to-bed heat transfer coefficient was found to be positively correlated with velocity and pressure, while negatively correlated with temperature within present experimental conditions. A dimensionless analysis was conducted using the Buckingham-theorem to derive the general form of an empirical wall-to-bed heat-transfer correlation in supercritical water fluidized bed for the Nusselt number, which was finalized based on the experimental data. What's more, the sold phase contributes more to the wall-to-bed heat transfer in SCW fluidized bed and the presence of solid phase enhances the heat transfer of SCW phase, which can be concluded from comparison of heat transfer between SCW fluidized bed and single phase of SCW. The experimental results and the proposed correlation in present work are useful for the design of SCW fluidized bed reactor.

Research on flow characteristics in supercritical water natural circulation: Influence of heating power distribution

Abstract There are many parameters that affect the natural circulation flow, such as height difference, heating power size, pipe diameter, system pressure and inlet temperature and so on. In general analysis the heating power is often regarded as a uniform distribution. The ANSYS-CFX numerical analysis software was used to analyze the flow heat transfer of supercritical water under different heating power distribution conditions. The distribution types of uniform, power increasing, power decreasing and sine function are investigated. Through the analysis, it can be concluded that different power distribution has a great influence on the flow of natural circulation if the total power of heating is constant. It was found that the peak flow of supercritical water natural circulation is maximal when the distribution of heating power is monotonically decreasing, minimal when it is monotonically increasing, and moderate at uniform or the sine type of heating. The simulation results further reveal the supercritical water under different heat transfer conditions on its flow characteristics. It can provide certain theory reference and system design for passive residual heat removal system about supercritical water.

Numerical study of flow and heat transfer of supercritical water in rifled tubes heated by one side

Deep understanding of flow and heat transfer of supercritical fluids in rifled tubes with non-uniform heating is of great importance for the safety of modern supercritical steam power equipment. In this paper, the heat transfer of supercritical water flowing in a vertical one-side heated rifled tube was numerically investigated. The differences of flow and heat transfer between the circumferentially non-uniform heated rifled tube (NRT) and uniform heated rifled tube (URT) were investigated. The distributions of temperature field, thermophysical properties, axial velocity and turbulent kinetic energy were analyzed. The mechanism of heat transfer improvement of NRT was comprehensively revealed. Results showed that the maximum inner wall temperature in NRT (Tiw,N,max) was 10 K lower than that in URT for the simulation conditions. Both radial and circumferential heat transfers were improved in NRT due to the swirling flow. Under high q/G (ratio of heat flux to mass flux) conditions, the buoyancy effect was weakened in NRT comparing to that in URT. Flow turbulence in boundary layer of y+ > 30 was intensified in NRT, which was crucial for supercritical fluid heat transfer. The positions of Tiw,N,max were not always at the midpoint of the heated side. And a prediction method for Tiw,N,max was proposed.

Comparison of heat transfer models with databank of supercritical fluid

Abstract Because of the large variation of thermal–physical properties near pseudo-critical point, heat transfer of supercritical water shows abnormal behavior. Thus, an accurate prediction of heat transfer between the cladding and fluid plays a very important role. It is necessary to investigate the reliably of heat transfer models in the vicinity of the pseudo-critical point. In the frame of evaluating the present HT models and developing new reliable prediction models, databank of heat transfer of supercritical water flowing in tube were established by Karlsruhe Institute of Technology (KIT) with more than 35,000 experimental data. A thorough analysis and assessment was carried out, to give an insight into the characters of the database. Experimental data from different sources are compared to figure out the deviation between different sources and to present the reliability of the database.

A new method on fluid-to-fluid scaling for heat transfer in tubes at supercritical pressures

Supercritical fluid heat transfer has great significance for the thermal-hydraulic design and safety performance of the SCWR (Super-Critical Water-cooled Reactor). Experimental studies on heat transfer at prototypical conditions of SCWRs would suffer a huge cost on safety and economy, which strongly limits the research of supercritical water heat transfer. One solution is to adopt model fluid technique which has been applied in thermal and fluids engineering widely. Therefore, finding the equivalent relationship between the experimental data in model fluid and those in prototype fluid becomes a key issue. It means a fluid-to-fluid scaling method is necessary for converting experimental data obtained by using model fluid into the prototype fluid. In the present study, from the perspective of thermo-physical properties similarity and conservation equations similarity, considering the comprehensive influence of the experimental conditions and fluid types, a theoretical study on fluid-to-fluid scaling is performed and most importantly, a new scaling method is proposed. The new scaling method adopts a modification on thermo-physical properties. Several dimensionless numbers are obtained according to the dimensionless momentum and energy governing equations. A new dimensionless number En is derived to present the comprehensive influence of experimental conditions and fluid types. According to the theoretical analysis, an entire set of scaling laws applied for supercritical fluid heat transfer in circular tubes was obtained. Based on the heat transfer data of supercritical R134a, the new scaling method was validated, indicating the feasibility and accuracy of the proposed new scaling method.

Experimental study of supercritical water heat transfer deteriorations in different channels

The experimental tests on heat transfer deterioration (HTD) of supercritical water flowing in 4 different kinds of channels, including tube, annular channel, and 2 × 2 bundle, have been carried out on the Supercritical WAter MUltiPurpose test loop (SWAMUP). The tube is made of Inconel alloy with different inner diameters; the annular channel consists of an Inconel alloy heated inner tube and a 304 stainless steel unheated outer tube. Two kinds of bundles were tested. The first kind of bundle consists of 4 Inconel-718 tubes with different Pitch-to-Diameter ratios, and the 2 × 2 bundle with grid spacers is installed into a square assembly box. The other kind of 2 × 2 bundle consists of 4 heater rods, and the rod bundle with wire wraps is also installed into a square assembly box. The heat transfer of supercritical water in simple channel like tube is stronger with larger cross-sectional flow area, but it is stronger with smaller cross-sectional flow area in complex channel like bundle. Two kinds of HTDs are observed in tube and annular channel, only the first kind of HTD is observed in bundle. The first kind of HTD occurs only at high ratio of heat flux to mass flow velocity in different channels, and the second kind of HTD is more likely to occur in simple channel which could be eliminated in bundle by the transverse turbulent flow. The effects of thermal–hydraulic and structural parameters on the second kind of HTD in tube are as follows: the HTD of supercritical water is severer with higher heat flux and the increment of wall temperature is much larger; the HTD occurs earlier and severer with lower mass flow velocity; the HTD delays and the wall temperature rises more slowly when the pressure rises; and the diameter has no evident effect on this kind of HTD. The heat transfer in bundles especially with wire wraps is qualitatively better and more stable than that in other tested channels. In order to avoid the first kind of HTD within the operating condition of SCWR, the low mass flow velocity condition and high heat flux condition should not occur at the same time during operation.

Consistency considerations on a large databank and wide range heat transfer prediction for supercritical water in circular tubes

The heat transfer coefficient (HTC) of supercritical water (SCW) shows abnormal behavior when compared with conventional fluids. This behavior is caused by steady variations of thermal–physical properties of SCW around the pseudo-critical point. A large database of heat transfer of SCW flowing in tube and rod bundles was proposed by the Karlsruhe Institute of Technology (Zhao et al., 2017). This databank contains 28,364 high quality experimental data points (nodes) for heat transfer of supercritical water flowing in tubes, covering a wide domain of flow parameters.Additional consistency checks were made in this work on the experimental data using our own developed procedure. This procedure was able to provide hints on the intrinsic reliability of the data and also error levels for almost all experimental nodes.The derived information on error levels made a high-performance methodology of data assimilation and model calibration (Cacuci and Ionescu-Bujor, 2010) subsequently accessible to this domain of experimental data. The BESTEST module (Badea et al., 2011) based on the methodology of data assimilation and model calibration (Cacuci and Ionescu-Bujor, 2010) was used in an iterative manner to optimize a new proposed correlation able to describe with good accuracy a large domain of the databank.

Numerical study on heat transfer of SCW near the pseudo-critical temperature in a hexagon sub-channel

A numerical study was carried out to study the local heat transfer performance of supercritical water (SCW) within a hexagon bare inner sub-channel by using the commercial CFD code CFX. The main objective was to focus on the special thermal–hydraulic characteristics near the pseudo-critical temperature (PCT) at different working conditions. The results show that the secondary flow induced in the inner sub-channel is weak in comparison with the axial main flow, and becomes weaker near the PCT. A stagnation point of the secondary flow is created right at the narrow gap, which can partially explain the weak heat transfer near the narrow gap region. The nonuniform geometry of the sub-channel dominates the creation of a secondary flow in the sub-channel near the PCT. The fluid temperature very close to the cladding surface dominates the heat transfer enhancement of SCW within the sub-channel. The maximum values of the local heat transfer coefficient appear when the PCT is within the buffer layer of that angular position, indicating that the buffer layer is directly related to the heat transfer enhancement. The system parameters evidently affect the heat transfer performance, and the corresponding local heat transfer performance varies differently in the circumferential direction.

Flow and heat transfer of supercritical water in the vertical helically-coiled tube under half-side heating condition

Flow and heat transfer of supercritical water in the vertical helically-coiled tube were investigated numerically using RNG k-ε model. The inner side of the coil was heated by constant heat flux and the outer side was adiabatic. Heat transfer performance under half-side heating is obtained and compared with that under uniform heating at the same heat transfer rate. Effect of specific heat, secondary flow velocity and buoyancy are discussed. The decrease of specific heat in the near-wall region, and the attenuated turbulence caused by the strong buoyancy force lead to a decrease in heat transfer for half-side heating. The deviations of heat transfer correlations on predicting half-side heating condition are evaluated, and a new heat transfer correlation for supercritical water in the vertical helically-coiled tube under inner half-side heating is proposed. The investigation into different statistical parameters shows that it is superior to the existing correlations.

Flow Instability in Parallel Channels with Water at Supercritical Pressure: A Review

Research into flow instability at both subcritical and supercritical pressures has attracted attention in recent years because of its potential of occurrence in industrial heat transfer systems. Flow instability has the potential to affect the safety of design and operation of heat transfer equipment. Flow instability is therefore undesirable and should be avoided in the design and operation of industrial equipment. Rahman et al. reviewed studies on supercritical water heat transfer with the aim of providing references for SCWR researchers. It was found out that most of the CFD studies and experimental studies were performed with single tube geometry due to the complexity of parallel channel geometry. Because studies performed with parallel channel geometry could provide detailed information to the design of the SCWR core, they called for more studies in parallel channel geometry at supercritical pressures in the future. In order to help understand how flow instability investigations are carried out and also highlight the need to understand flow instability phenomenon and equip the designers and operators of industrial heat transfer equipment with the needed knowledge on flow instability, this study carried out a review of flow instability in parallel channels with water at supercritical pressures.

Heat transfer of supercritical water in annuli with spacers

Experimental investigations on heat transfer of supercritical water in vertical annular channel equipped with spacers have been carried out at the SWAMUP-II facility in Shanghai Jiao Tong University. The test section consists of a heated outer rod of 15.0 mm inner diameter and a concentric unheated inner rod of 7.41 mm outer diameter. The simplified spacers with blockage ratio of 0.2 and 0.3 are equipped into the annular channel. The water flows upward cooling the heated tube. The pressure and mass flux ranges are 23–25 MPa and 450–1200 kg/(m2·s), respectively. Moreover, the heat flux ranges from 400 to 1000 kW/m2, and the fluid temperature is in the range of 240–450 °C. The heat transfer behaviors downstream from the spacers are analyzed. The effects of seven parameters on the heat transfer enhancement downstream from the spacer are presented. At a certain location downstream from the spacer, the further heat transfer impairment may occur on the basis of the previous deterioration. Conventional heat transfer enhancement prediction methods are compared with the experimental data. The correlations based on the subcritical conditions show the limitations at the supercritical pressure. An improved correlation has been derived.

Heat transfer regimes for a flow of water at supercritcal conditions in vertical channels

Heat transfer regimes observed in experiments with water at supercritical conditions flowing in vertical channels of various cross-sections (such as round pipes, annulus, or rod bundles) are analyzed. In accordance with the established practice, the normal and the deteriorated heat transfer regimes were singled out as the basic regimes specific for heat carriers with highly variable properties. At the same time, it has been established that most published experimental data on supercritical pressure water heat transfer along the length of test sections demonstrate combined (or transient) heat transfer regimes. The features can be presented as a superposition of characteristics of the above-mentioned basic regimes. The combined regimes are not stable in certain ranges of water flow conditions in which sudden transitions between the basic regimes can occur. A system of similarity criteria governing heat transfer rate in the vicinity of the critical point is examined. As applicable to cores of water-cooled reactors, due to a small hydraulic diameter of cooling channels, buoyancy forces acting in these channels are negligible as compared with the inertia effects caused by thermal acceleration of the flow and viscous force. This concept yields two integrated criteria whose use in the correction factors for the basic heat transfer equation, which we proposed previously for the normal regimes, adequately (with an error of 20–25%) describes the specific of the heat transfer coefficient in the normal, deteriorated, and combined regimes. A system of equations is proposed for design calculation of heat transfer in channels of nuclear reactors cooled with supercritical pressure water.

Experimental study on heat transfer of supercritical water flowing upward and downward in 2 × 2 rod bundle with wrapped wire

Heat transfer to supercritical water flowing upward and downward in a 2×2 rod bundle with wrapped wire was experimentally investigated. The rod bundle consists of four heating rods within out-diameter of 10mm. The rods are arranged in a 2×2 square array and the pitch-to-diameter ratio is 1.18. The bundle is inserted into a square assembly box with round corners. The water as coolant flows upward or downward in the channel between the rod bundle and the assembly box. The pressure ranges from 23 to 26MPa, mass flux ranges from 450 to 1200kg/(m2s), heat flux ranges from 200 to 1000kW/m2, and the fluid temperature ranges from 200 to 450°C. The effects of system parameters including mass flux, heat flux and pressure on the heat transfer are presented, respectively. The heat transfer deterioration occurs at a low G/q ratio in the upward flow, but does not appear in the downward flow. Watts-Chou correlation and Bishop correlation give the best predictions when evaluated against the experimental data both in the upward and downward flow. The experimental data are compared with the bare rod bundle. The comparison indicates that the swirl flow of wrapped wire promotes the heat transfer in the rod bundle.

CFD analysis of heat transfer in hexagonal subchannels of super fast reactor in upward flow

AbstractA super fast reactor is a fast spectrum, supercritical, water‐cooled reactor. This paper represents CFD analysis of heat transfer in hexagonal subchannels of super fast reactor using FLUENT in ANSYS. The numerical simulation of grid stability was done by considering different mesh sizes and the turbulence model for heat transfer of supercritical water was also carried out and compared with the experimental data. RNG k‐ϵ turbulence model with enhanced wall treatment was considered for simulations. Heat transfer and heat generation rate analysis of the outer surface rod wall is carried out with different subchannels by changing various parameters like boundary conditions and pitch‐to‐diameter ratio. The analyses reveal that the outer surface of the rod wall temperature decreases with increase in pitch‐to‐diameter ratio. Maximum coolant temperature rises in edge subchannels more than corner subchannels. Further analysis is carried out with different mass fluxes. Increases in mass flux has minimal effect on the maximum rod wall surface temperature. Maximum cladding surface temperature for the corner subchannel is less compared to the edge subchannel. Heat generation rate also decreases with increase in pitch‐to‐diameter ratio. This paper also investigates the buoyancy effect on subchannels with varying heat flux as boundary conditions considering constant mass flux.

Nonuniform heat transfer of supercritical water in a tight rod bundle – Assessment of correlations

The heat transfer coefficient of supercritical water in a rod bundle is essential for the fuel design of the Supercritical Water-Cooled Reactor (SCWR). Although numerous correlations have been proposed over the past few decades to predict the heat transfer coefficient, the conclusions are inconsistent due to the limited experimental data obtained in fuel bundle. In the present paper, 20 correlations were assessed against the experimental data obtained in a tight 2×2 rod bundle. Circumferential maximum wall temperature and minimum heat transfer coefficient were selected as the benchmark data to get a conservative conclusion in support of the fuel design and safety analysis. The assessments showed that the performances of these correlations vary greatly depending on the mass flux and heat flux. Most correlations give reasonable Nusselt numbers at normal and enhanced heat transfer regimes, but become worse when the heat transfer is impaired. Comparison of these correlations against the total of 714 data indicated that the correlation proposed by Chen-Fang is the best with an average error of −0.44% and a standard deviation of 6.4%. All of the experimental Nusselt numbers were successfully predicted within ±20% error band.

Supercritical water heat transfer coefficient prediction analysis based on BP neural network

14 groups of experimental data have been collected for determining the heat transfer coefficient within the scope of supercritical water pressure. On the basis of experimental data, the BP neural network predictive model has been built for the determination of supercritical water heat transfer coefficient. Using the BP neural network prediction model, a study has been conducted to determine effect of changing certain parameters such as heat flux, mass flux, pipe diameter and pressure on the heat transfer coefficient of supercritical water. The prediction results show that the mean error, standard deviation and the root mean square error are 0.179032187%, 4.128897482% and 0.179032187% respectively, the regression coefficient R is 0.97165 and the maximum error is 16.06332%. The trained BP neural network prediction model can applied for better prediction and understanding of the heat transfer coefficient of supercritical water. The prediction range is as follow: specific enthalpy is 451.30–3135.87kJ/kg, mass flux is 400–3000kg/m2s, heat flux is 200–2960kW/m2, pressure is 22.6–31MPa and diameter is 0.7–38.1mm.

Analysis of entropy generation behavior of supercritical water flow in a hexagon rod bundle

The numerical simulation was carried out to investigate the local entropy generation in turbulent mixed convection heat transfer of supercritical water within the inner sub-channel of SCWR. The SSG Reynolds stress model with enhanced wall treatment was used as a turbulence model. The circumferential heterogeneity of heat transfer was introduced into the entropy generation analysis. The mechanisms of entropy generation in the near wall region within the boundary layer and the region away from the wall were discussed in detail. The results show that the buffer layer has great influence on heat transfer enhancement, and both the viscous sub-layer and buffer layer act significantly in the entropy generation with respect to circumferential heterogeneity. Usually, the entropy generation in the narrow gap is relatively larger than that in the sub-channel center when comparing the contribution of entropy production caused by heat transfer to that caused by energy dissipation. The simulation findings may provide a theoretical direction for the conceptual design of SCWR.

Experimental investigation on transient heat transfer in 2 × 2 bundle during depressurization from supercritical pressure

The experimental investigation on transient heat transfer of supercritical water during depressurization in 2×2 bundle with wire wraps has been performed on the SWAMUP-II co-constructed by CGNPC and SJTU. The test section consists of two channels separated by a square steel assembly box with round corners. Water flows downward in the first channel and then turns upward in the second channel to cool the 2×2 bundle with wire wraps installed inside the assembly box. The bundle consists of four heater rods of 10mm in O.D. arranged with pitch-to-diameter ratio of 1.18. Experimental parameter ranges cover pressure from 16 to 26MPa, mass flux from 850 to 1450kg/m2s, heat flux from 250 to 650kW/m2, inlet fluid temperature from 345 to 365°C, and depressurization rate of 1 and 2MPa/min. The experimental data are obtained and heat transfer characteristics during depressurization from supercritical to sub-critical conditions are discussed. The fraction of heat transfer from inner channel to outer channel is lower than 15% under the test conditions. Four different transient heat transfer phenomena during depressurization are observed. The boiling crisis is likely to occur in SCWR during depressurization as the operating condition of SCWR is compared with the test condition. The boiling crisis is less likely to occur with the increase of mass flux, and more likely to occur with the increase of heat flux or inlet fluid temperature. The wall temperature increment during boiling crisis is smaller with higher mass flux, and larger with higher heat flux or inlet fluid temperature. The depressurization rate affects the transient heat transfer itself trivially.