Circumferential Wall Temperature Research Articles

Numerical investigation of thermal-hydraulic parameter distribution characteristics during dryout evolution in the helically coiled once-through steam generator

A two-fluid three-flow-field model that considered interactions among the droplets, steam and liquid film during dryout evolution in the helically coiled once-through steam generator was introduced to numerically study the distribution of the thermal-hydraulic parameters. Simulation results showed that the slip ratio first increased and then decreased in the flow boiling process. The slip ratio was relatively small due to the secondary flow in the tube, and the maximum value was only 1.065. Because the steam had a relatively strong entrainment effect on the droplets, the frequency of droplets hitting the wall was high, resulting in only a small level of deviation from thermodynamic equilibrium. When dryout occurred, the local wall temperature at different circumferential angles, as well as the average wall temperature, reached a maximum and then decreased. The dryout incipience occurred at a distance of 2.8 m from the inlet of the helically coiled tube. The circumferential angle at the position of dryout incipience was 100–140° while the dryout region at the outlet extended to 70–270°. The circumferential wall temperature was non-uniformly distributed and its maximum value was first increased and then decreased.

The numerical simulation of enhanced heat transfer on a Linear Fresnel molten salt-type receiver tube filled with porous media

In this paper, three-dimensional numerical simulation was taken on a Linear Fresnel solar receiver tube using molten salt as heat transfer fluid (HTF), in which the porous media was filled to enhance the heat transfer efficiency. The simulation was to analyze the influence of the different conditions (filling rate, porosity and thermal conductivity) on heat transfer effect and wall temperature difference. The results revealed that the Nu (Nusselt number) increased firstly and then decreased with the increasing filling rate in both center filling and annular filling types. The optimal thermal performance were obtained when filling rate were 0.8 and 0.2 in center filling and annular filling, respectively. The Nu were about 1.7 and 1.5 times as the clear receiver. The circumferential temperature difference decreased firstly and then increased with filling rate increasing in both center filling and annular filling types. The lowest circumferential temperature differences were achieved at the filling rate 0.8 and 0.4 in center filling and annular filling types, and temperature difference decreased 15.88°C and 22°C compared with clear receiver, respectively. The Nu and PEC both decreased with porosity increasing. However, the thermal conductivity of porous media had little influence to the Nu and circumferential wall temperature.

Experimental and CFD estimation of single-phase heat transfer in helically coiled tubes

Numerical simulation of helically coiled tubes has been reported by many researchers, while few researchers have taken buoyancy into account and their detailed numerical results have not been compared with experimental data. In this article, Computational Fluid Dynamics (CFD) simulations are carried out for single-phase flow in vertically oriented helically coiled tubes applying Reynolds Stress Model with gravity in consideration. The simulation results reveal that flow and heat transfer inside helically coiled tubes are influenced by both centrifugal force and buoyancy. Circumferential wall temperature distribution can be predicted well, and the peak value moves in anti-clockwise direction from the top to the inner side of the coil with centrifugal force increased. The Nusselt number along the periphery at cross section is also presented at various boundary conditions. The deviations of heat transfer coefficient between CFD values and experimental results are within ±25%. Hence, the numerical method stated in this article can be used to predict single-phase heat transfer in helically coiled tubes.

Experimental investigation on partially filled liquid pools under combined thermal and vibrational loads

The seismic excitation affects the performance of a heat exchanger submerged in a water pool. Hence, the potential effect of external vibrations on a boiling and heat transfer system is investigated in this work. The effect of horizontal vibration on heat transfer performance of a submerged heater rod has been experimentally investigated under atmospheric pressure conditions. The experimental results show that the heat transfer is enhanced by 15% with vibration in single-phase convection. Heat transfer in subcooled boiling is significantly affected by higher mode excitation frequencies. However, there is no significant influence of excitation frequency on heat transfer in the saturated boiling regime. The effect of circumferential wall temperature around the heater is investigated and a maximum temperature difference of 6.3% has been found between the top and bottom heater surface. Thermal stratification of pool water is also studied without and with external vibration. The vertical temperature difference between zone 1 and zone 2, (i.e. thermal stratification) is suppressed by 80% at excitation frequency of 6.35 Hz.

Modeling the influence of tube support plates on flow boiling and dryout of once-through steam generators

Considering the tube support plates installed among the tube bundle of actual once-through steam generator, the mathematical model suitable for smooth channel was modified to numerically simulate the two-phase flow and heat transfer and dryout under three structures (no tube support plates, tube support plates with and without gaps). The results show that the slip ratio increases rapidly when fluid flows through the tube support plates, and then decreases rapidly under the strong disturbance caused by the turbulence transition at the downstream position of tube support plates. The maximum slip ratio for the case of TSPs with gaps is lower than that for the other two cases, which is conducive to prevent interphase slipping. The turbulence transition and great steam quality caused by heat transfer enhancement at the downstream position of tube support plates both have positive effects on tearing the annular liquid film, resulting in the dryout position being at the upstream position of that for the case of no tube support plates. The damage of dryout deterioration can be relieved significantly when considering tube support plates, and the corresponding rising amplitude of wall temperature at dryout position reduces from 300 K for the case of no tube support plates to 200 K. With the development of flow and heat transfer, the non-uniform levels of circumferential wall temperature for the cases of tube support plates with and without gaps show completely opposite trends.

Experimental investigation on heat transfer behavior in a tight 19 rod bundle cooled with supercritical R134a

The strong non-uniform distributions of circumferential wall temperature and heat transfer coefficient will exist in tight-lattice rod bundles. However, there is still a lack of understanding and only a few experimental data are available. In this paper, experimental investigations are performed on circumferential heat transfer behavior in a tight hexagonal 19 rod bundle using supercritical R134a as working fluid. The rod surface temperatures at various circumferential positions along the axial flow direction are measured. A defined local hydraulic diameter is introduced to represent the heterogeneity of channels in the tight bundle. The circumferentially non-uniform distribution of wall temperature is found to be strongly related to the variation of the local hydraulic diameter. In normal heat transfer pattern, the circumferential temperature gradient is large especially in the region far away the pseudo critical point, but becomes small near the pseudo critical point due to a strong enhancement of heat transfer coefficient. Parametric effects of pressure, mass flux and heat flux on circumferential non-uniformity of wall temperature inside the rod bundle are also analyzed.

Heat transfer distribution in helical coil flow boiling system

The objective of the present work is to study the heat transfer distribution in the helical coil flow boiling system. Infrared thermal imaging technique is used to measure the local wall temperature. The wall temperature is measured across the circumference and in axial direction along the length of a helical coil. The literature showing circumferential heat transfer distribution in a helical coil during flow boiling process is scarce. The present work compares the wall temperature and heat transfer coefficient distribution on the inner side and the outer side of helical coils at different diameter ratios and density ratios. In the present study, the experimental databank (containing total 400 heat transfer coefficient data points) includes the data not only from the present experimental study but also from the literature. The data bank includes the subcooled and the saturated flow boiling data. The data includes two fluids namely water and R123. Data covers the range of parameters namely a density ratio of 30–1600, a mass flux of 100–1300 kg/m2 s, a heat flux of 2–640 kW/m2, an exit quality of −0.5 to 1 and a coil to tube diameter ratio of 14–58. Ten empirical correlations which include well referred correlations of straight tubes and available correlations of helical coils are evaluated with the databank. The study concluded that the circumferential wall temperature variation in helical coils decreases with increase in a diameter ratio and with decrease in a density ratio. The circumferential averaged heat transfer coefficient during a saturated flow boiling in a helical coil is same as a straight tube. A correlation is suggested to measure the circumferential average heat transfer coefficient in helical coils.

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.

Experimental study on gas-liquid flow heat transfer in a horizontal tube with wire-coil inserts

The forced evaporative heat transfer of the non-boiling CO2-water flow in a horizontal tube with low water mass-flow rate enhanced by wire-coil inserts was examined through experiments. Simultaneously, a method for providing CO2-water flow is proposed. The results show that inserting wire coil can improve the performance of the gas-liquid heat transfer since the liquid can continuously wet the entire perimeter of the tube under the swirl force produced by the helical coils; hence, the circumferential wall temperature is more uniform than in smooth tubes when the gas/liquid mass-flow rate is relatively low. Increasing either the gas or liquid mass-flow rate can enhance the heat transfer, but the enhancement efficiency decreases with the rise of the mass-flow rate. An optimum liquid flow rate exists, which depends on the heat flux and the gas flow rate. When the liquid flow rate is larger than the optimum, the heat transfer barely improved. The mean heat-transfer coefficients increase approximately 2.5–3.5 times over those of the single-phase flow in a smooth tube, but they are influenced by the gas/water flow rate and wall heat flux. The traditional method of summarizing the heat-transfer coefficients [h=q/(tw-tf)] seems unsuitable in this situation.

Heat transfer characteristics of supercritical water in a 2 × 2 rod bundle – numerical simulation and experimental validation

In this paper, heat transfer of supercritical pressure water in a 2 × 2 rod bundle was analyzed numerically. Three turbulence models were evaluated against experimental data. Comparisons showed that the SSG Reynolds stress model gives the best prediction for normal and enhanced heat transfer modes. Heat transfer of supercritical water in the rod bundle was studied with respect to the variations of mass flux, heat flux and pressure. Non-uniform distributions of bulk temperature exist on the cross-section of the flow channel, and the temperature gradient becomes small as the bulk temperature approaches the pseudo-critical temperature. Two heat generation methods from the rods, i.e. constant wall heat flux and internal heat source, were examined in the present analyses. Numerical results showed that the average heat transfer is nearly the same for the two methods, but the circumferential wall temperature gradient is significantly lowered when conjugate heat transfer is taken into consideration. The effects of pitch-to-diameter ratio (P/D) on heat transfer were also analyzed. Among the assessed four P/D ratios, the average heat transfer at the P/D of 1.2 is the best, and the circumferential wall temperature gradient is reduced with increasing P/D.

Comparison of simplified heat transfer models and CFD simulations for molten salt external receiver

In the absence of experimental correlations of the solar external receiver performance, it is particularly necessary to develop thermal models to optimize the receiver operating modes and to properly design such equipments. Since CFD models require an enormous computational cost to simulate a receiver, two simplified bi-dimensional implicit-scheme models have been developed. Both models consider axial and circumferential variations on the heat flux absorbed by the tubes and on the wall temperature. One assumes homogeneous surface boundary condition and the other assumes homogeneous surface heat flux at the discretized tube wall.The effects of mass flow rate, and wind velocity on the receiver performance have been analysed considering steady state operation. The results have been compared with a simulation carried out with ANSYS Fluent. Both simplified models are able to predict the heat fluxes, the salt and the tube wall temperature with a deviation lower than 6% compared to CFD simulations. The analysis of the developed models has been also compared with a model usually employed that does not consider variable circumferential temperature. The results show that keeping constant the circumferential tube wall temperature leads to lower wall temperature, underestimating the film temperature, the thermal stress and the salt decomposition.

Role of gaseous film cooling injector orientation in duct flow: Experimental Study

Experimental investigations are carried out to analyze the effect of coolant injector configuration on overall film cooling performance in a cylindrical test section similar to a rocket combustion chamber. Three different injector orientations are investigated: (i) holes parallel to core gas flow, (ii) holes at a tangential angle to the core gas flow, and (iii) compound angle holes inclined to the wall both in the tangential and azimuthal direction. The objective is to provide a consistent set of measurements for cooling effectiveness within the same test facility with respect to different blowing ratios ranging from 1.69 to 3.9. The results suggest that coolant injection parallel to the axis should be used in situations where far field effectiveness is of concern. The tangential injection, in general, shows lower wall protection compared to other schemes. The results show that the compound angle configuration augments film cooling effectiveness in the near injection regimes. However comparison of the effectiveness values for the compound injectors suggests the existence of an optimum compound angle configuration. The circumferential wall temperature data represent systematic changes in the coolant flow behavior with the change from straight injection to compound angle injection.

Computational fluid dynamics investigation of thermal–hydraulic characteristics for a steam generator with and without tube support plates

A three-dimensional computational fluid dynamics model with the thermal phase change model is used to investigate the thermal–hydraulic characteristics of a steam generator with and without quatrefoil tube support plates. The two types of modeled designs are a unit pipe with and one without tube support plates. The computational fluid dynamics simulations capture the boiling phenomena, vortex and recirculation distributions, and the periodic characteristics of the circumferential wall temperature in the regions surrounding the tube support plates. The cross-flow energy responsible for flow-induced vibration damage in the region of the U-bend tubes is obtained with the aid of these localized thermal–hydraulic distributions. A comparison between the key parameters of the unit pipe models with and without tube support plates clearly reveals the influence of tube support plates in guiding flow behavior and alleviating flow-induced vibration damage for a steam generator’s U-bend tube bundle. Therefore, this computational fluid dynamics model can provide technical support for optimizing tube support plate design and improving the thermal–hydraulic characteristics of steam generator.

Heat transfer of air‐water dispersed flow in a vertical pipe

Heat transfer of air-water dispersed flow in a vertical heating pipe and its enhancement have been studied. The axial and circumferential wall temperature distributions were measured using various mist ratios and wall heat fluxes. The measured wall temperature increased sharply at a particular streamwise location, with a notable variation in the circumferential profile. This sharp increase was conceivably caused by a breakdown of the water film rather than by its dryout. A separate unheated experiment was carried out to estimate the droplet deposition velocity and the water-film flow rate. A numerical analysis, taking into account heat and mass transfer from the water film to the bulk flow, was performed in order to estimate the mean wall temperature. Good agreement was obtained with the experimental results in the area where the entire inner surface of the pipe was covered with the water film. In this area, the rate of heat transfer was approximately seven times larger than that for single phase air flow. This enhancement was shown to be due mainly to evaporation of the water film. The mechanism of heat transfer enhancement is discussed in detail using the numerical analysis results. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 255–270, 1998

Heat transfer of air-water dispersed flow in a vertical pipe

Heat transfer of air-water dispersed flow in a vertical heating pipe and its enhancement have been studied. The axial and circumferential wall temperature distributions were measured using various mist ratios and wall heat fluxes. The measured wall temperature increased sharply at a particular streamwise location, with a notable variation in the circumferential profile. This sharp increase was conceivably caused by a breakdown of the water film rather than by its dryout. A separate unheated experiment was carried out to estimate the droplet deposition velocity and the water-film flow rate. A numerical analysis, taking into account heat and mass transfer from the water film to the bulk flow, was performed in order to estimate the mean wall temperature. Good agreement was obtained with the experimental results in the area where the entire inner surface of the pipe was covered with the water film. In this area, the rate of heat transfer was approximately seven times larger than that for single phase air flow. This enhancement was shown to be due mainly to evaporation of the water film. The mechanism of heat transfer enhancement is discussed in detail using the numerical analysis results. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 255–270, 1998

Heat Transfer of Air-Water Dispersed Flow in a Vertical Pipe.

The heat transfer and its enhancement of an air-water dispersed flow in a heated vertical pipe have been studied. The axial and circumferential wall temperature distributions were measured for various mist ratios and wall heat fluxes. The measured wall temperature increased sharply at a particular point in the direction of the stream with a notable variation in the circumferential profile. This sharp rise was thought to be caused by the breakdown of the water film rather than by its dryout. A separate experiment in which no heating was used was carried on to estimate the droplet deposition velocity and the water-film flow rate. Numerical analysis taking into account the heat and mass transfer from the water film was performed to estimate the mean wall temperature, and good agreement was obtained with the experimental results in the area where the entire inner surface of the pipe was covered with the water film. In this area, the rate of heat transfer was approximately seven times higher than that for a single phase air flow. This enhancement was shown to be mainly due to the evaporation of the water film. The mechanism of the heat transfer enhancement is discussed in detail using the numerical analysis results.

Two-dimensional wall temperature measurements and heat transfer enhancement for top-heated horizontal channels with flow boiling

Two-dimensional (circumferential and axial) wall temperature distributions were measured for top-heated coolant channels with internal geometries that include smooth walls, spiral fins and both twisted tape and spiral fins. Freon-71 was the working fluid. The flow regimes studied were single-phase, subcooled flow boiling, and stratified flow boiling. The inside diameter of all test sections was near 10.0 mm. Circumferentially averaged heat transfer coefficients at several axial locations were obtained for selected coolant channels for a volumetric flow rate of 4.738 x 10 −5m 3/s, 0.19 MPa (absolute) exit pressure, and 22.2°C inlet subcooling. Overall (averaged over the entire channel) heat transfer coefficients were compared for the various channel geometries. This comparison showed that the channel with large-pitch spiral fins had higher heat transfer coefficients at all power levels. However, the results appear to indicate that if the twist ratio (ratio of the twisted tape period to the inside diameter) is decreased, the configuration employing both fins and a twisted tape will have had greater enhancements.

Dryout and Wall Temperature Fluctuations in Helically Coiled Evaporating Tubes.

An experimental study on dryout characteristics of helically coiled tubes was conducted by measuring wall temperature fluctuations near the dryout point. The region where the wall temperature fluctuations were observed in the coiled tube was wider than that in a straight tube. Circumferential wall temperature differences near the dryout point became significant when the coil diameter was small and the mass flow rate was low. The dryout mechanism in the coiled tube was discussed on the basis of the flow pattern map constructed by the author's air-water experimental results. Critical heat fluxes in helically coiled tubes, which were higher than that in a straight tube, were compared with empirical correlations. Comparing the present results with Katto's correlation for a straight tube, a modified correlation was presented.

FULLY DEVELOPED LAMINAR HEAT TRANSFER IN CIRCULAR-SEGMENT DUCTS WITH UNIFORM WALL TEMPERATURE

Heat transfer to constant-property, fully developed, laminar flows in circular-segment ducts with uniform wall temperature (T) has been analyzed. Besides representing a compact surface, the segment duct geometry models the flow cross section of a circular tube with a straight-tape insert. Two variations in the T thermal boundary condition are considered: constant axial and circumferential wall temperature, and constant temperature on the curved surface but an adiabatic flat wall. These two conditions model the extremes of the fin effects of a straight-tape insert, i.e., 100% and zero fin efficiencies, respectively. Numerical solutions, obtained by using finite difference techniques, are presented for both the velocity and temperature fields. The isothermal friction factors are in excellent agreement with analytical solutions reported in the literature. The Nusselt number results for the two thermal boundary conditions are presented for different segment shapes, 0° ≤, 6 ≤, 90°, and they represent the lower...

Effect of variable viscosity in the presence of variable wall temperature on condensation on a horizontal tube

Variable viscosity solutions have been obtained taking account of both radial and tangential temperature variation. Solutions have been obtained for ethylene glycol, which has a strong dependence of viscosity on temperature, and for both free and forced convection. For uniform surface temperature, constant-property solutions, with an appropriate reference temperature, give accurate results for the mean heat-transfer coefficient, even where the viscosity varies across the condensate film by a factor of around 10. Constant-property solutions are much less accurate in the presence of appreciable circumferential wall temperature variation. The theoretical results are compared with heat-transfer measurements for condensation of ethylene glycol.