Heat Transfer Deterioration Research Articles

Evaluation Research on Identification Methods of Heat Transfer Deterioration of Supercritical Co2

Evaluation Research on Identification Methods of Heat Transfer Deterioration of Supercritical Co2

Characterisation of Observed Heat Transfer Deterioration Modes at Supercritical Pressure with the Aid of a Cfd Model

Characterisation of Observed Heat Transfer Deterioration Modes at Supercritical Pressure with the Aid of a Cfd Model

Characterisation of Observed Heat Transfer Deterioration Modes at Supercritical Pressure with the Aid of a Cfd Model

Characterisation of Observed Heat Transfer Deterioration Modes at Supercritical Pressure with the Aid of a Cfd Model

Numerical Investigation on Deteriorated Heat Transfer of Supercritical Water Flowing Upward in Tubes with Variable Cross-Sectional Geometries

Numerical Investigation on Deteriorated Heat Transfer of Supercritical Water Flowing Upward in Tubes with Variable Cross-Sectional Geometries

On the effect of pseudo-condensation on the design and performance of supercritical CO[formula omitted] gas chillers

Supercritical CO2 is used as a work fluid in both heat pump and power cycles. As a fluid at supercritical pressure is heated or cooled, it may undergo a smooth transition from a liquid-like state to a gas-like state or vice versa. This transition, during which the thermophysical properties vary sharply with temperature, can be referred to as pseudo- boiling or condensation. Using both analytical and numerical methods, it is shown that pseudoboiling theory helps to understand how the unique heat transfer characteristics of a supercritical fluid affect heat exchanger performance and design, in particular a gas chiller. Due to pseudo-condensation, classical approaches such as the ε−NTU and LMTD methods fail when rating or designing a sCO2 gas chiller. Using the heat of pseudo-condensation, the heat exchanger can be regarded to consist of a pre-cooler, condenser and a super-cooler. By further dividing the pre-cooler and super-cooler into two parts and subsequently applying the ε−NTU method per part yields very good results with respect to both the prediction of required size and entropy generation for various operating parameters. The influence of pseudo-condensation is reduced at higher pressures and is negligible when the structural energy required for the transition from liquid-like to a gas-like state is smaller than the required thermal energy required. It is shown that the local effectiveness of the condenser part is reduced (more so than the other parts) when the heat capacity ratio RC is varied from unity to less than unity, leading to enhanced irreversibility due to pseudo-condensation. Furthermore, the enhanced and deteriorated heat transfer regime (such as when a sCO2 downward flow is cooled) lead to significantly different required heat exchanger sizes. Finally, through the use of Monte Carlo simulations, it shown that the uncertainty of a Nusselt correlation complicates designing heat exchangers in which pseudo-condensation occurs. The simulations show that heat exchangers should be 50% larger than the size that is predicted using a Nusselt correlation if the design performance is to be ensured.

Heat Transfer Limitations in Supercritical Water Gasification

Supercritical water gasification (SCWG) is a promising technology for the valorization of wet biomass with a high-water content, which has attracted increasing interest. Many experimental studies have been carried out using conventional heating equipment at lab scale, where researchers try to obtain insight into the process. However, heat transfer from the energy source to the fluid stream entering the reactor may be ineffective, so slow heating occurs that produces a series of undesirable reactions, especially char formation and tar formation. This paper reviews the limitations due to different factors affecting heat transfer, such as low Reynolds numbers or laminar flow regimes, unknown real fluid temperature as this is usually measured on the tubing surface, the strong change in physical properties of water from subcritical to supercritical that boosts a deterioration in heat transfer, and the insufficient mixing, among others. In addition, some troubleshooting and new perspectives in the design of efficient and effective devices are described and proposed to enhance heat transfer, which is an essential aspect in the experimental studies of SCWG to move it forward to a larger scale.

РАСЧЁТНЫЙ МОДУЛЬ ДЛЯ ОПРЕДЕЛЕНИЯ ФИЗИЧЕСКИХ ПАРАМЕТРОВ В КАНАЛЕ РЕАКТОРА С ТЕПЛОНОСИТЕЛЕМ ЗАКРИТИЧЕСКОГО ДАВЛЕНИЯ

The paper describes the software module FRC-SCP intended for thermohydraulic simulation of a coolant under supercritical pressure (SCP) and a cooled fuel rod. Several designs of supercritical water reactors utilize the transition of the coolant from the pseudoliquid state to pseudogas while it is heated in the reactor core. SCP coolant under pseudophase transition exhibits specific behavior as its density changes significantly. Furthermore, coolant thermophysical properties (density, heat capacity, viscosity, thermal conductivity) can also vary across the coolant channel affecting heat transfer from the fuel rod to the coolant and consequently the fuel temperature. Existing feedback dependencies on coolant density and fuel temperature are important for the nuclear safety analysis of the reactor. The paper considers the present version of the FRC-SCP module. It implements the steady-state thermohydraulic channel solver to calculate coolant parameters: temperatures of the flow core and a heater, as well as coolant pressures, densities etc. User-specified correlations define the heat transfer law under the normal conditions. The module solves the thermal problem for the fuel rod consistently with the channel thermohydraulic problem. It is also possible to couple the FRC-SCP module with the neutron physical codes. Thermohydraulic module is tested against experiments on the heat transfer to SCP water in heated tubes. We discuss the behavior of the fuel cladding under conditions imitating the deteriorated heat transfer modes and the effect of the nuclear fuel thermal conductivity.

Turbulence model assessment and heat transfer phenomena inside a rectangular channel under forced and mixed convection

A very high-temperature gas-cooled reactor (VHTR) is an attractive option for hydrogen production due to its high operating temperature. The reactor cavity cooling system (RCCS), a passive safety system of the VHTR, surrounds the reactor vessel with rectangular riser channels and removes decay heat from it. Under emergency operation conditions such as a decrease in the flow rate of the RCCS, the flow condition inside the RCCS riser can become a mixed convection condition accompanying heat transfer deterioration. To design and operate the VHTR with reliable safety, accurate predictions for various convective heat transfer conditions in RCCS riser channels are required. In this study, by comparing the results of an air flow visualization experiment, the predictability of the turbulence models was assessed for the convective heat transfer and complex flow characteristics near the corner region of the rectangular duct confirmed from the experimental study. In this paper, the heat removal rate through the test section of the visualization experiment was quantified under various convective heat transfer conditions, and local temperature measurement data from the experiment were presented to describe the heat transfer deterioration phenomena inside a rectangular riser in turbulent mixed convection. Linear and nonlinear eddy viscosity models were evaluated by comparing the heat transfer coefficient, local temperature, local flow structure including secondary flow, and turbulence quantities obtained from the experiment. Furthermore, improvements to turbulence models for enhancing the prediction of mixed convection heat transfer in rectangular riser ducts were proposed.

Unsteady stagnation point flow past a permeable stretching/shrinking Riga plate in Al2O3-Cu/H2O hybrid nanofluid with thermal radiation

PurposeThe investigation of fluid flow and heat transfer is incredibly significant in the present era, particularly in the engineering and manufacturing industries. Hence, this study aims to concern with analysing the unsteady stagnation point flow towards a permeable stretching/shrinking Riga plate of Al2O3-Cu/H2O. The effect of thermal radiation on the boundary layer flow is also taken into account.Design/methodology/approachThe multi-variable differential equations with partial derivatives are transformed into third-order and second-order differential equations by applying appropriate transformations. The reduced mathematical model is solved in the MATLAB system by using the bvp4c procedure. This solution approach is capable of producing multiple solutions once the necessary assumptions are provided.FindingsThe results of various control parameters were analysed, and it has been observed that raising the solution viscosity from 0% to 0.5% and 1% improves the coefficient of skin friction and thermal conductivity by almost 1.0% and 1.9%. Similar response and observation can be witnessed in the addition of modified Hartmann number where the highest values dominate about 10.7% improvement. There is a substantial enhancement in the heat transfer rate, approximately 1.8% when the unsteadiness parameter leads around 30% in the boundary layer flow. In contrast, the increment in thermal radiation promotes heat transfer deterioration. Further, more than one solution is proven, which invariably leads to a stability analysis, which validates the first solution’s feasibility.Originality/valueThe present results are new and original for the study of flow and heat transfer on unsteady stagnation point flow past a permeable stretching/shrinking Riga plate in Al2O3-Cu/H2O hybrid nanofluid with thermal radiation.

Flow and Heat Transfer of Supercritical CO2 in a Vertical Tube Under Ocean Rolling Motion

Abstract This paper explores the fluid flow and heat transfer behaviors of supercritical carbon dioxide (SCO2) in a tube. The application is utilization of waste heat from a marine gas turbine. The effects of ocean rolling motion on the thermofluid characteristics of SCO2 in a circular tube are numerically investigated based on a verified turbulence model. It is found that the time-averaged heat transfer capacity over a rolling period is improved over static conditions by 7.9%. However, the onset of heat transfer recovery is postponed, and the range of heat transfer deterioration is extended. Under the action of inertial forces due to the rolling motion, heat exchange between cooler/denser and warmer/lighter fluids is enhanced. Secondary circulation is formed when t/tc = 0.325, and the section-averaged heat transfer coefficient is improved by a maximum of 71%. For various periods, a parabolic can be distinctly found in terms of the variation trend of time-averaged heat transfer coefficient, which behaves differently from conventional fluids. Regarding the instantaneous thermal performance, a polarization phenomenon can be observed under severe rolling. With the rise of the layout height, the time-averaged heat transfer performance of the tube increases monotonously, and the maximum increment is 10.64% in studied range.

Thermomechanical analysis of a lead-cooled fast reactor with a fast-running model

The aim of this work is the development and testing of a simplified and fast-running model for thermomechanical analysis of a Lead-cooled Fast Reactor (LFR). For this work a fast-running mechanical model has been coupled with a heat transfer model previously developed, as part of AZTLAN-platform project. In a nuclear reactor, an optimal heat transfer in the fuel is needed to avoid unwanted phenomena related to sudden increases in temperature. One of the main mechanisms of deterioration of heat transfer in the fuel is the thermal expansion of the fuel and cladding since it leads to the narrowing or even blockage of the space between them causing thermal and mechanical loads that can produce fuel failure. Here, the results of the numerical calculation for stress, deformation, and radial displacement in the fuel of an LFR are presented as a function of the temperature. The simplified model was validated with a Finite Element Method peridynamics model getting consistent results.

Heat transfer by unstable solution having the lower critical solution temperature

In this article, we present new data on the non-stationary heat transfer by partially-soluble binary liquids in the course of deep penetration into the region of their metastable and unstable (short-term superheated with respect to the diffusion spinodal) states. The object of research was an aqueous solution of polypropylene glycol-425 having the lower critical solution temperature (LCST). In the course of pulse experiments, the superheating degree reached 200 K at the heating rate of 105 K/s. The main heating mode was the constant power mode with a characteristic pulse length of 10 ms. The pressure, being the parameter changeable in a series of measurements, was varied from 1 to 100 MPa. Upon reaching a certain superheating degree, a significant enhancement of heat transfer (EHT) has been revealed. We associate this result with the spinodal decomposition, which is the natural relaxation mechanism in the considered part of the phase diagram. An unexpected result was the detection of the deteriorated heat transfer stage, preceding the stage of EHT. These stages reflect the characteristic features of the spinodal decomposition at the given conditions. The significant effect of EHT confirms the prospects of using the solutions with LCST as coolants. A model of heat transfer during the decay of an unstable solution has been developed. It is based on an analysis of the nucleation, growth and motion of a new phase domain in the field of temperature gradient. The simulation results are consistent with the results of experiments under conditions of powerful heat release and small temporal and spatial scales.

3-D numerical study of subcooled flow boiling in a horizontal rectangular mini-channel by VOSET

A three-dimensional conjugated numerical simulation is conducted to investigate subcooled flow boiling in a horizontal rectangular mini-channel based on the interface capturing method (VOSET). A reasonable nucleation density site model based on experimental results is adopted. Hundreds of bubbles with different sizes are successfully captured. Simulation results reproduce typical processes of subcooled boiling flow: growth, coalescence, detachment, and condensation of bubbles. And different flow boiling patterns, including pseudo bubbly flow, bubbly flow, slug flow, and pattern transition, are visualized under different heat fluxes. Furthermore, the effects of heat flux and vapor fraction on heat transfer coefficient are discussed in detail. Heat transfer deterioration is observed for high heat flux because of the formation of dry patches, and the complexity of heat transfer performance varying with vapor fraction is found. Based on those, the heat transfer mechanisms of subcooled flow boiling at different heat flux in the present mini-channel are discussed.

Heat transfer deterioration and visualized flow state of supercritical CO2 in a vertical non-circular channel

An experimental investigation on heat transfer deterioration and the discussion of visualized flow states for supercritical CO2 flow in a non-circular channel were here proposed. A significant blurred shadow, induced by local density variations, was photographed in near heated wall region as heat transfer deterioration occurred. The blurred shadow qualitatively showed the special flow changes of supercritical fluids. The effects of system parameters were studied and the heat transfer correlations were evaluated by obtained experimental data. Three Dittus-Boelter type correlations showed better predictions, while the semi-empirical correlation with physical meaning showed the worst. Combined with the analysis of buoyancy, a modified heat transfer correlation for present experiments was suggested and it was compared with that for tube flows. The experimental findings and visualized flow state may further strengthen the understanding on supercritical heat transfer and extend the researches of supercritical fluids in non-circular channels.

Molecular dynamics simulations on evaporation of a suspended binary mixture nanodroplet

Mixture working fluids have the advantages of active design and adjustable physical properties, therefore it has a promising future in many power and refrigeration applications. Studies on the evaporation process of mixture nanodroplets are helpful to reveal the mechanism of heat transfer at the interface of mixture working fluids. However, at present, there are few studies on the evaporation of mixture droplet, and the conclusions on the pure fluids evaporation cannot be directly applied to the mixture working fluids, thus it is necessary to carry out specific research on the evaporation of mixtures. In this study, molecular dynamics simulation was used to study the evaporation process of R32/R1234yf nanodroplets with different compositions (xR32=0, 0.2, 0.4, 0.6, 0.8, 1) in a large space. The dynamic changes of evaporation rates of droplets with different components were compared, and the evaporation characteristics of droplet with different concentrations were analyzed in combination with the density, temperature and concentration field distribution near the liquid-vapor interface during the evaporation process. The results show that the evaporation rate of the mixture nanodroplets is lower than the linear interpolation with those of the two components, which is similar to the heat transfer deterioration of the mixture working fluids in pool or flow boiling. The prediction deviation of R32/R1234yf mixture nanodroplet evaporation by the D2 law is around 80%. Considering the scale effect and the interfacial concentration gradient, the prediction accuracy of the D2 law can be improved.

The empirical characteristics on transient nature of Al2O3-water nanofluid pool boiling

The present investigation focuses on assessing the transient characteristics of nucleate pool boiling heat transfer in Al2O3-water nanofluids. The stable nanofluid samples were prepared for the wt.% concentrations of 0.01, 0.1, and 1. The experiments were performed for the 12 h (approx.) boiling durations with varying heat flux and nanofluid concentrations. The surface temperatures remained relatively constant for low and medium heat flux with varying nanofluid concentrations. The high heat flux studies presented steady, oscillatory & incremental, and sudden temperature spikes for low, medium, and high concentration nanofluids. The heat transfer deterioration of up to 90% and nanoparticle deposition increment of 50% to 300% was observed with increasing heat flux, nanofluid concentrations, and boiling durations. The microscopic images' qualitative analyses showed the evolution of nucleation sites and deposition patterns over varying trends. The empirical characteristic curves were proposed to summarise the transient effect for varying heat fluxes and nanofluid concentrations.

Boiling Heat Transfer and Visualization for R717 in a Horizontal Smooth Mini-tube

The paper presents an experimental and theoretical investigation on the two-phase flow boiling heat transfer with ammonia (R717) as the refrigerant in a horizontal smooth mini-tube with inner diameter of 3mm. The experimental data were obtained in the following condition, the heat flux density is 10∼30 kW · m − 2 , the mass flux is 40∼200 kg · m − 2 · s − 1 , the saturation temperature is -10∼10 ℃, and the range of vapor quality is 0.1 ∼1. The experimental results show that the heat transfer coefficient before dryout phenomenon increases and gradually reaches a peak with the increase of vapor quality, but occurs heat transfer deterioration after dryout phenomenon. Combined with the transform of flow patterns, the effect of mass flux, heat flux density, and saturation temperature on the heat transfer coefficient and friction pressure drop are analyzed. The theoretical gas phase flow velocity of ammonia is much higher than other refrigerants, and the flow boiling heat transfer for ammonia is more dependent on convective boiling. The existing correlations of two-phase flow are compared with the measured data of ammonia. According to the experimental and comparative results, Kew and Conwell correlation and Müller-Steinhagen and Heck correlation were updated respectively, and new boiling heat transfer correlation and frictional pressure drop correlation were obtained.

Experimental investigation on flow boiling heat transfer of R134a in a minichannel heatsink with copper foam fins

Metal foam is widely used for heat-transfer enhancement. Flow boiling of R134a in a minichannel heatsink with copper foam fins was studied. The heat sink contained four channels that were 503 μm in width and 990 μm in depth, and the copper foam fins had a porosity of 0.8. The mass flux in the experiment ranged from 595 to 1213 kg/m2 s, and the heat flux was in the range of 22–172 W/cm2. Heat transfer coefficients under different mass flow rates, heat fluxes, and qualities were obtained. The heat transfer deterioration phenomenon at different heat fluxes was analyzed. Compared with a solid-fin minichannel heat sink, the heat transfer was enhanced at low heat fluxes, whereas it was weakened at high heat fluxes. The turning point heat flux was between 87 and 100 W/cm2 and increased as the flow rate increased. Mechanisms of the heat transfer deterioration in the copper foam minichannel heatsink were explained by comparing flow patterns in the two heatsinks at the high heat flux.

Dual-effect evaluation of heat transfer deterioration of supercritical carbon dioxide in variable cross-section horizontal tubes under heating conditions

Heat transfer characteristics analysis of supercritical carbon dioxide (sCO2) in variable cross-section tubes is critical for its promotion and application. This paper proposed three horizontal heat exchange tubes, i.e., straight tube, converging tube, and diverging tube. On this basis, we used numerical simulation to analyze the convective heat transfer performance of sCO2 in the horizontal tube with variable cross-section under heating conditions. The diverging tube effectively enhances the overall heat transfer performance compared with the uniform cross-section tube, and the heat transfer rate is increased by 19.26% compared with that of the straight tube. In contrast, the converging tube weakens the heat transfer ability. Moreover, we proposed quasi-air film and eddy blockage concepts to reveal the fluid's heat transfer deterioration mechanism near the top bus. Wherein the quasi-air film is full of sCO2, which has a low thermal conductivity close to that of air at room temperature, and generates near the top bus. Besides, the criterion factor, But was proposed by double corrections of density and temperature for judging the occurrence of eddy blockage, and the Nu-based evaluation shows that the eddy blockage can deteriorate the heat transfer performance at the top of the horizontal tube by 17%. The physical mechanism of heat transfer deterioration under the heating condition is clarified by the dual-effect of quasi-air film and eddy blockage on this account.

Neutronic and Thermal-Mechanical Coupling Schemes for Heat Pipe-Cooled Reactor Designs

Abstract Space fission power systems can enable ambitious solar-system and deep-space science missions. The heat pipe-cooled reactor is one of the most potential candidates for near-term space power supply, featuring safety, simplicity, reliability, and modularity. Heat pipe-cooled reactors are solid-state and high-temperature (up to 1500 K) reactors, where the thermal expansion is remarkable and the mechanical response significantly influences the neutronics and thermal analyses. Due to the considerable difference between heat pipe-cooled reactors and traditional water reactors in the structure and design concept, the coupling solutions for light water reactors cannot be directly applied to heat pipe-cooled reactor analyses. Therefore, a new coupling framework and program need to consider the coupling effects among neutronics, heat transfer, and mechanics. Based on the Monte Carlo program rmc and commercial finite element program ansysmechanicalansys parametric design language (APDL), this work introduces the three coupling fields of neutronics (N), thermal (T), and mechanics (M) for heat pipe cooled reactors. Besides, the finite element method and the Monte Carlo program use different meshes and geometry construction methods. Therefore, the spatial mapping and geometry reconstruction are also essential for the N/T-M coupling, which is discussed and established in detail. Furthermore, the N/T-M coupling methods are applied to the preliminary self-designed 10 kWel space heat pipe cooled reactor. Coupling shows that the thermal-mechanical feedback in the solid-state reactor has negative reactivity feedback (about −2000 pcm) while it has a deterioration in heat transfer due to the expansion in the gas gap.