Occurrence Of Heat Transfer Deterioration Research Articles

Experimental Research on the Flow and Heat Transfer Characteristics of Subcritical and Supercritical Water in the Vertical Upward Smooth and Rifled Tubes

Experiments were conducted to investigate the heat transfer and flow characteristics of the vertical upward smooth and rifled tubes from subcritical to supercritical pressure. The distributions of wall temperature and heat transfer coefficient (HTC) were obtained, and the HTC correlations and friction resistance coefficient correlations were fitted with experimental data. In addition, the influences of heat flux and type of tube on heat transfer performance were analyzed. The research shows that heat flux has different influences on the heat transfer characteristics under different pressures. The increase in heat flux improves the heat transfer characteristics in the nucleate boiling region, yet it leads to the advance in heat transfer deterioration. However, for supercritical water, the increase in heat flux reduces the heat transfer ability. In addition, using the rifled tube not only improves the heat transfer performance, but also inhibits the occurrence of heat transfer deterioration. The fitted correlations have great predictive ability for the heat transfer coefficient and friction resistance coefficient, and the average relative fitting errors are limited to 20%.

Numerical study of convective heat transfer to supercritical CO2 in vertical heated tubes

Supercritical heat transfer is important to design and operate advanced power generation system. Non-uniform heating is frequently encountered such as for sCO2 boiler and solar receiver applications. Here, 3D numerical simulations are performed in turbulent regime, coupling solid wall and sCO2 in tubes. Non-uniform heating is simulated by applying heat via a thin metal layer at the outer tube wall, covering half circumference of tube wall. Calculation results matched experimental data well. Heat transfer is compared between uniform heating and non-uniform heating. Numerical results are re-processed by the pseudo-phase transition theory, including a liquid-like region in tube core and a gas-like layer near the tube wall. The two layers of structure are interfaced by a terminating temperature T+ which is determined by thermodynamics. Hence, supercritical heat transfer is evaluated based on the relative importance of thermal resistances induced by the liquid-like region and the gas-like film region. It is found that wall temperatures for uniform heating are higher than those for non-uniform heating, which is explained by larger thermal resistance of the gas-like film for uniform heating than that for non-uniform heating. This difference ensures better heat transfer for non-uniform heating than uniform heating. We conclude that in turbulent regime, the gas-like film dominates the magnitudes of wall temperature and the occurrence of heat transfer deterioration for both uniform and non-uniform heating. Our work is important for heat transfer facilities in supercritical domain, especially under non-uniform heating condition.

Experimental investigation and theoretical analysis on the heat transfer deterioration of supercritical CO2 in mixed and forced convection in vertical-straight tube with downward flow

The heat transfer deterioration of supercritical fluids in vertical-upward flow has been extensive studied since 1930s. However, few studies focused on the heat transfer deterioration of supercritical CO2 in vertical-downward flow. In this paper, the sudden rise of wall temperature for supercritical CO2 in mixed convection with vertical-downward flow is observed firstly in the experiments, except for in forced convection. The effect of operation parameters on the heat transfer deterioration are studied, covering pressures (7.5∼9 MPa), mass flow rates (240∼1120 kg/(m2•s) and heat fluxes (45–200 kW/m2). A new shear stress redistribution model is proposed by considering the thermal acceleration in the boundary layer with the convection term of motion equation. The new model is verified with multiple sets of independent experimental data from the public literatures. The effect of thermal acceleration not only includes the acceleration in the average mass bulk, but also has the conspicuous acceleration of fluids in the boundary layer. The experimental results demonstrate that the occurrence of heat transfer deterioration in vertical-downward flow is mainly induced by the effect of thermal acceleration in the boundary layer.

Heat Transfer and Secondary Flow Characteristics in a Horizontally Round Pipe for Cooling a Scramjet Combustor by Supercritical n-Decane

Abstract To figure out the abnormal flow characteristics and thermal performance of supercritical fluids, some detailed information of supercritical pressure n-decane flowing in a horizontally round pipe is studied in terms of secondary flow induced by the huge density change or buoyancy force. According to an evaluation of turbulence models, the shear stress transport k–ω is suitable to execute the case of horizontal flow. It is observed that the temperature distributions between the upper wall region and the lower wall region are asymmetric and the location of the maximum buoyancy force coincided with the position of Tpc (pseudo-critical temperature). The generation of a rotating flow arising from the heated wall determines the occurrence of heat transfer deterioration (HTD). In the boom stage of the HTD phenomenon, a dead zone that is close to the upper wall was formed due to the influence of vortices. In contrast, the maximum buoyancy force is located in the core flow zone and it forces the fluid in the mainstream to participate in the cooling process of the heated wall. In addition, the dead zone in the vicinity of the upper wall is broken. This is the main reason why heat transfer deterioration could be inhibited effectively.

Flow and heat transfer performance of supercritical pressure carbon dioxide in pipes with discrete double inclined ribs

Heated flow of supercritical pressure carbon dioxide in pipes with discrete double inclined ribs (DDIR) were numerically studied in this work. The turbulent flow and heat transfer was solved by a modified Shear Stress Transport k-ω model, where a variable turbulent Prandtl number formulation was incorporated. The model accuracy was verified by experimental data covering wide working conditions. Numerical runs were performed at 7.58 MPa, mass flux of 200 ~ 800 kg /(m2•s), and heat flux of 56.7 kW/m2. Results show that the strong buoyancy effect existing in smooth pipes was heavily inhibited by DDIR and no serious deterioration occurred. Sensitivity analysis on geometrical parameters suggest that the rib spacing the most sensitive one to the occurrence of heat transfer deterioration. The influence of inclination angle was much weaker than the rib height and spacing. The overall thermal performance of the DDIR pipe was the best when Bo* was around 4.5 × 10−6, with a PEC of 3~4. In the regions with very strong or negligible buoyancy, however, PEC was a much lower value between 1~2. The major role of discrete ribs is to interrupt the continuous development of near-wall buoyant layer and subsequently eliminate the negative effect of buoyancy on heat transfer. Ribs cannot effectively increase the positive effect of buoyancy on heat transfer. Finally, Bo* was found to be appropriate for the performance evaluation and optimization of enhancement techniques for supercritical heat transfer.

Turbulent heat transfer characteristics of supercritical n ‐decane in a vertical tube under various operating pressures

To explore the effect of the operating pressure on the flow and thermal characteristics in a vertical tube with supercritical n-decane, the inner wall temperature along the streamwise direction, the applicability of empirical correlations and buoyancy criteria are studied firstly. The mechanisms in views of density distribution, velocity field, turbulence intensity, the thickness of the thermal boundary layer, and secondary flow are then analyzed. It is observed that the buoyancy is helpful for the phenomenon of heat transfer deterioration at lower operating pressures but higher operating pressures can diminish this tendency. According to this reason, the empirical correlation proposed by Bae and Kim is suitable for the higher operating pressures but cannot well predict the occurrence of heat transfer deterioration. However, it can be evaluated by the buoyancy criterion Gr/Re2 = 0.01 qualitatively. The decrease of turbulence intensity, the thickened thermal boundary layer, and secondary flow generation make contributions to the heat transfer deterioration in particular. Similarly, this situation can be diminished and even removed by the higher operating pressures.

Numerical study of effects of vortex generators on heat transfer deterioration of supercritical water upward flow

The occurrence of heat transfer deterioration (HTD) in supercritical pressure boilers, where the wall temperature rises abruptly is an undesirable phenomenon that is limiting the design of new promising engineering applications. This may cause operational problems such as tube burnouts which could result in a catastrophic system failure. Understanding ways of eliminating HTD is of great importance for the design of safe and reliable supercritical pressure boilers. In this paper, vortex generators (VGs) are installed in circular and annular channels to investigate HTD phenomenon of supercritical water flowing upward at high heat flux and low mass flux using shear stress transport (SST) k-ω turbulence model in ANSYS-FLUENT. Results on wall temperature distribution for the smooth and enhanced channels are compared with available experimental data, and good agreements are obtained. The effects of VG’s size, position, and number on HTD are investigated. The results indicate that VG’s size slightly suppresses and delays HTD downstream of the VG, and there exists an optimum size beyond which HTD starts to aggravate. A strong effect of VGs on heat transfer coefficient (HTC) profiles is observed at locations where VGs are installed and at the downstream locations, with the normalized HTC showing that the wall temperature oscillates a couple of times before becoming stable once the flow returns to fully developed state. The VG’s position has the most significant effect on HTD for any single VG. Increasing the number of VGs installed in-line at the start of the major peak baselines significantly suppresses and delays the peak. Mechanism analysis based on radial distributions of velocity and turbulent kinetic energy (TKE) at different axial positions of both channels shows that HTD suppression is caused by the weakening of the buoyancy effect due to increased TKE near the wall downstream of the VG, whereas the delay is caused by the boundary layer recovery effect due to flow redevelopment after being disrupted by the VG.

On assessment of heat transfer deterioration of a channel with supercritical n-decane for scramjet engines cooling

To explore the local heat transfer deterioration (HTD) behavior in cooling channels with the endothermic hydrocarbon fuel for a scramjet engine, the flow and thermal performance of a rectangular channel cooled by supercritical hydrocarbon fuel are analyzed in detail, especially at the region of the heated wall. A 3D geometrical model is simulated under a SST k-ω turbulence model to validate results which agrees well with available experimental data. The critical variations of turbulent kinetic energy and representative thermo-physical properties are selected to assess the internal mechanism of HTD in terms of the deteriorative point of the wall temperature and the heat transfer coefficient. This study reveals that the occurrence of HTD is strongly affected by the variation of turbulent kinetic energy, and that the local HTD behavior is considered to be induced by the laminar flow caused by thermal acceleration effect.

Numerical investigation of supercritical water flow in a vertical pipe under axially non-uniform heat flux

The authors conducted a numerical investigation of water flow in a vertical pipe during the transition from sub-to supercritical conditions when non-uniform power is applied axially at the wall. The Computational Fluid Dynamics calculations were first assessed with experimental results found in literature, where uniform heat flux was applied. Next, the model was used to perform calculations of four various cases with non-uniform heat flux distribution along the axis, however, the average value is equal to the value of the reference axially uniform-heated case. Results showed high dependency of the wall temperature distributions on the shape of the power distribution curve applied at the wall. It was found that if the peak value of heat flux is close or behind occurrence of heat transfer deterioration (HTD) phenomenon, the wall temperature increases rapidly to higher than in the reference case. However, if the peak is moved to the front part of the pipe, it is possible to achieve lower temperatures than when uniform heat flux was applied. Calculations of convective heat transfer i.e. Nusselt number show that currently, only two correlations were able to follow the trend of the results i.e. the Bishop's and Ornatsky's formulas. From the sensitivity calculations, it was found that the wall temperature is much more dependent on the mass flux than the operating pressure since the temperature variation is much higher and it is followed by the change of the position where HTD occurs.

Numerical study of heat transfer deterioration of turbulent supercritical kerosene flow in heated circular tube

Turbulent flow and convective heat transfer of supercritical kerosene flow in an axisymmetrically heated circular tube with a diameter of 2mm and at a mass flow rate range of 0.0015–0.015kg/s and a wall heat flux range of 0.15–2.0MW/m2 are numerically studied using Reynolds averaged Navier–Stokes method with a two-layer turbulence model. The thermophysical and transport properties of kerosene are determined by a 10-species surrogate with the Extended Corresponding State method. Mesh dependency is first investigated and numerical results of fuel and wall temperatures are compared with experimental data for validations. The results show that flow properties such as velocity and Reynolds number increase significantly along the axial direction as the fuel temperature rises and kerosene undergoes the state transition from liquid to supercritical state. Deterioration of convective heat transfer is found to occur when the wall heat flux exceeds a critical value and at the same time, the wall temperature approaches the pseudo-critical temperature of kerosene. The present results show that deterioration of heat transfer are attributed to the development of turbulent properties in the near-wall region based on the results of turbulent kinetic energy and turbulence production term. The relation between the critical heat flux (qwc) for occurrence of heat transfer deterioration and the mass flux (G) is studied and a fitting formula of qwc and G is obtained.

Conditions for the occurrence of heat transfer deterioration in light hydrocarbons flows

Heat transfer deterioration could affect heated channel flows of supercritical pressure fluids in a number of technological applications. Aim of the present study is to analyze the phenomenon onset and to investigate its dependence on the pressure, for three light hydrocarbons: methane, ethane and propane. To this goal a parametric numerical analysis is carried out on uniformly heated straight channels varying, for each different species, the inlet reduced pressure and the enforced heat flux. A parabolized Navier–Stokes solver is used to carry out the simulations, together with Helmholtz energy equation of state and accurate models for the transport properties. Results lead to an unambiguous definition of the threshold value for the ratio between heat flux and specific mass flow rate which identifies the boundary for heat transfer deterioration onset. On the basis of this definition, for assigned inlet reduced temperature, a correlation for the threshold parameter in terms of reduced pressure is presented for the considered species.

Prediction of unstable behaviour in a heated channel with water at supercritical pressure by CFD models

The paper presents the results of the application of computational fluid dynamics in the prediction of unstable behaviour in heated channels containing supercritical fluids. The work is conceived to extend and discuss previous work made with one-dimensional codes, taking profit of the greater detail provided by CFD. In particular, a single channel with cross section area and heating power similar to those proposed for typical supercritical reactor core subchannels is addressed. To search for unstable behaviour, constant pressure drop boundary conditions are imposed across the flow duct and the heating power is slowly increased up to the point at which inlet and outlet flow rates are seen to oscillate, with a pattern typical of density wave instabilities. Two different turbulence models are adopted, being the standard k– ε model equipped with wall functions and a low-Reynolds number model. The effect of inlet and outlet singular pressure drops is also assessed. Recently introduced dimensionless numbers are adopted to define the threshold of the unstable behaviour. Comparison with the results from one-dimensional models is also made to ascertain the level of agreement or discrepancy. These first results on the prediction of instability in heated channels at supercritical pressure by CFD also provide information about the possibility of cyclic occurrence of heat transfer deterioration and restoration during flow oscillations.