Circumferential Temperature Gradient Research Articles

Heat transfer characteristics of horizontal flow boiling with uniform and non-uniform circumferential heat flux

This study assesses effects of the non-uniform circumferential heat flux on horizontal flow boiling of R141b in a stainless-steel tube at ambient system pressure with an inner diameter of 10.92 mm. The main motivation for this are application in Concentrated Solar Power using the Direct Steam Generation process inside horizontal absorber tubes. Experiments were conducted for two mass velocities (50 and 150 kg m−2s−1) and two mean heat fluxes (3.4 and 11.0 kW m−2), with three different heating conditions (uniform, top-concentrated, and bottom-concentrated). The differences in the Onset of Nucleate Boiling between uniform and bottom-concentrated heating are assessed, and the transient behavior of the wall temperatures is investigated. It is found that the heating condition significantly affects the mean heat transfer coefficient in the case of non-annular flow regimes but practically not in annular flow. The circumferential wall temperature gradient is with uniform heating highest for non-annular flow regimes. But with non-uniform heating the circumferential wall temperature gradient is highest for annular flow. The use of empirical correlations for the heat transfer coefficients are evaluated.

Numerical investigation of the overall thermal and thermodynamic performance of a high concentration ratio parabolic trough solar collector with a novel modified twisted tape insert using supercritical CO2 as the working fluid

Supercritical CO2 (sCO2) is favoured as the next-generation working fluid for advanced high-temperature and high-concentration ratio parabolic trough solar collectors (PTSC) due to the fluid’s low cost, availability and thermal stability at high temperatures. However, the lower thermal transport properties of sCO2 compared to conventional working fluids necessitate heat transfer enhancement of receivers using sCO2. In this study, a combined and extensively validated approach using Monte-Carlo ray tracing for optical analysis and computational fluid dynamics (CFD) for thermal and thermodynamic analysis were used to investigate the overall thermal and thermodynamic performance of a high concentration ratio sCO2-based PTSC with a modified twisted tape (MTT) inserts in the receiver. MTT inserts with twist ratios in the range of 2.0 to 4.0 and width ratios in the range of 0.75 to 0.95 were investigated. Results show enhancement of the heat transfer performance by up to 73% and thermal efficiency by 6%, and reduction of the receiver heat losses and circumferential temperature gradients when the MTT inserts are used. In addition to presenting results using the first law of thermodynamics, this study also considered the second law of thermodynamics by evaluating the entropy generation rates due to heat transfer and fluid friction irreversibilities. Results showed that the use of MTT inserts caused a reduction in the total entropy generation rates, with the lowest irreversibility occurring at a Reynolds number of 1,650,000 for a twist ratio of 2.0 and width ratio of 0.95.

Improved model for thermal transmission in evacuated tubes: Effect of non-uniform heat flux and circumferential conduction

Evacuated tubes are extensively utilized for converting solar energy into thermal energy owing to their exceptional thermal properties and affordability. The circumferential temperature gradient of the evacuated tube is significant to the heat and mass transfer inside, but it is often neglected. This research proposed an improved model incorporating non-uniform heat flux and circumferential conduction to obtain the temperature distribution. In the experiment, A thermal insulation material was inserted inside the inner tube to create an adiabatic boundary. Various conditions (non-uniform, rectangular, and uniform heat fluxes, circumferential conduction of the coating) were numerically compared. The enhanced combined thermal transmission model was validated, demonstrating a relative error of 6.45 % in temperature increase calculation and increasing the prediction accuracy by 42.15 % at least. The distribution of the heat flux and the thermal conduction of the coating were identified as the primary factors affecting the heat transfer properties of the evacuated tube. An imbalanced distribution of heat flux resulted in non-uniform heat transfer, whereas the circumferential conduction within the coating effectively modulated the temperature distribution to achieve uniformity. The local heat transfer coefficient exhibited non-uniformity, with calculated values ranging from 0.23 to 3.25 W/(m2∙K), and an error range of −16 to +3 %.

Redesign of a Disc-on-Disc Computer Numerical Control Tribometer for a Wide-Range and Shudder-Resistant Operation

The paper presents a redesign of the custom disc-on-disc-type tribometer intended for the experimental characterization of the friction and wear of automotive dry clutch friction lining. The redesign is aimed at expanding the operating range at which the machine is not sensitive to shudder vibrations. This is achieved through a set of hardware and software upgrade measures. First, the natural frequency of the normal load-generation linear axis of the machine is increased by enlarging its bending stiffness and reducing the suspended mass. The former is realized by replacing the single, two-axial force/torque piezoelectric sensor with a set of three three-axial piezoelectric force sensors, adding a set of stiff linear guides, and reducing the lengths of the cantilevers of lateral forces acting on the linear axis guide system. The latter is accomplished by reducing the overall dimensions of the cooling disc and redesigning the thermal insulation components. The shudder sensitivity resistance is further reduced through individual normal force-readings-based adjustment of parallelism between friction contact surfaces and the increase in the stiffness of eccentrically positioned water-cooling pipes. Finally, the stability of the coefficient of friction and, consequently, the wear process are boosted by adjusting the control routines to minimize the circumferential and/or radial temperature gradients. These adjustments include the introduction of a clutch lock-up interval at the end of the clutch closing cycle, a minimum cooling delay inserted between two closing cycles, and maximum normal force demand of the clutch torque controller. The performance gain of the upgraded tribometer is demonstrated through a study of the dry clutch friction plate static wear experimental characterization for a wide range of operating conditions.

Numerical and experimental study on thermal performance and thermal stress of molten salt receiver during preheating

Due to the high thermal stress, the evacuated molten salt tower receiver is prone to safety problems during the daily preheating, but there is relatively little research on it, especially on on-site receivers. In this study, the preheating performance of a lab-scale receiver is experimentally investigated first, and the influence of several operating or environmental factors on its thermal performance is obtained. The experimental results indicate that the tube front side always reaches the wall temperature requirement of salt circulation before the tube back side. Then, the numerical model of the receiver tube based on the finite volume method and 2D thermoelastic method is established and verified. Finally, by applying the numerical model to an on-site receiver, the influence of extreme environmental conditions on the preheating performance of the on-site receiver are obtained. It is found that the minimum and maximum thermal stresses occur at 90° and 180° of the outer wall, respectively, and the thermal stress is determined by the circumferential temperature gradient. The equivalent thermal stress first increases and then decreases during preheating, and the maximum equivalent thermal stress reaches within the first few minutes. Compared with the windless condition, the maximum equivalent thermal stress under strong wind (10 m/s) is reduced by 20.5%. The maximum equivalent thermal stress is linearly related to the ambient temperature, and it decreases by 6 MPa for every 10 °C increase in ambient temperature.

Performance study of a free-piston Stirling heat pump with a circumferential temperature gradient in the heating heat exchanger

The free-piston Stirling heat pump (FPSHP) is efficient, environmentally friendly and has a wide available temperature range, making it a promising device for building heating. The temperature of the external heat-carrying fluid is different at the inlet and outlet of the heating heat exchanger, so there is a temperature gradient in the heating heat exchanger. The larger the scale of the FPSHP, the more pronounced the temperature gradient. However, the influence of such a temperature gradient on system performance has not been investigated so far. This paper therefore presents a detailed study of the FPSHP with a circumferential temperature gradient in the heating heat exchanger. The calculation results show that a cascade output of energy can be achieved when there is a circumferential temperature gradient in the heating heat exchanger. With the same outlet temperature, the larger the temperature difference between the inlet and outlet of the heating heat exchanger, the higher the COP. However, due to the mixing of internal working gases with different temperatures in the compression space, part of the high temperature heat is directly transferred to the low temperature module and output at a lower level, adversely affecting system efficiency. Fortunately, this adverse effect is found to be insignificant when compared with multiple independent small FPSHPs without circumferential temperature gradients. Considering the higher investment cost of multiple small FPSHPs, a single large FPSHP is also a good choice for large heating capacity demand.

Experimental and numerical study on the PHWR channel heat-up under an oxidizing environment

The loss of primary as well as emergency coolant flow causes the convective cooling of the fuel bundle to degrade, raising the temperature of the fuel channel. Secondary heat released by the Zr/steam chemical reaction heats the channel even more intensely. The present work examines the thermal behavior of a heated channel in an oxidizing environment. An experimental system was created to simulate the late phase of the accident scene. The Pressure Tube (PT) in the channel was configured to contact the Calandria Tube (CT) at the bottom, simulating sagging deformation. The simulation was performed using ANSYS Fluent software. The modeling of the effect of the Zr/steam chemical reaction was done by adding a heat source to the cladding. A comparison was made between the obtained results and the available results of the same channel configuration in a non-oxidizing environment. It was found that significant axial, as well as circumferential temperature gradients were developed in the channel compared to that obtained under a non-oxidizing environment.

Studies of thermal stratification stresses and its impact on fatigue design of NPP piping

Thermal stratification is identified as one of the causes of failure in many nuclear power plant piping. It causes high circumferential temperature gradient in horizontal pipe portions and results in high thermal stresses. In general, most of the piping design codes do not consider these stresses. This paper presents a detail study of thermal stratification stresses and its impact on fatigue life of horizontal portions of NPP piping. The finite element evaluations of thermal stratification stresses are carried out for different constraint conditions at pipe end and for several idealizations of circumferential temperature variation. Simplified equations are developed for calculation of thermal-stratification induced thermal axial and tangential stresses. The thermal stresses evaluated using these developed formulas are compared with the FEM and the test data available in literature. A case study was carried out on a thermally stratified typical NPP surge line. Here the thermal stresses and their impact on fatigue damage was evaluated.

The influence of four-wire structure on the flow and heat transfer process in supercritical water-cooled reactor fuel assembly

Based on the supercritical water-cooled reactor fuel assembly, a 19-rod bundle physical model with a hexagonal outer casing is established, and the four-wire structure is added to the bare bundle. To analyze the influence of wire spacers on the flow and heat transfer process, numerical investigations of supercritical water in the reactor under design conditions are performed by using the Reynolds stress model with enhanced wall treatment. The results show that the mass redistribution is the main factor affecting the distribution of thermal–hydraulic parameters regardless of the existence of wires. The wire plays a role in weakening the mass redistribution by reducing the cross-flow between the adjacent sub-channels. However, due to the large local hydraulic diameter of the side sub-channels and the sharp change of the supercritical water physical properties, the fluid gradually gathers in the external sub-channels. In addition, the four-wire structure drives the fluid to spiral upwards around the fuel rod, which significantly enhances the convective heat transfer between the fluid and the fuel rod. At the same time, the influence of the local hydraulic diameter on the circumferential non-uniformity is weakened by the wires, and the circumferential wall temperature gradient of the external fuel rod is significantly reduced.

Experimental and Numerical Investigation into Temperature Distribution of a Simulated PHWR Coolant Channel Under Heatup Condition

The postulated dual-failure accident, i.e., loss of primary coolant flow along with impairment of the emergency coolant injection system, leads to peak fuel temperatures. It is well known that the temperature of the fuel assemblies is one of the significant factors that affect the outcome of an accident. Therefore, the present work aims to thoroughly investigate the thermal response of a single channel under postulated accident conditions. An experimental system was developed to capture the steady-state heat and temperature distribution in a representative 37-element fuel channel for a decay heat of 6.13 kW. Ohmic heating of the fuel rod simulators (FRSs) mimicked the generation of radioactive decay heat. Numerical simulation was also performed using the Fluent 19.1® code, and the discrete ordinates method was used to solve the radiative transfer equation. Based on the experimental results and the simulation results, it was found that the maximum Zircaloy-4 cladding temperature ≈850°C to 870°C was in the center ring. The temperature was found to vary around the circumference for each of the FRSs. Furthermore, the outer ring FRSs that had the lowest temperature developed the highest circumferential temperature gradient. In the pressure tube, the average circumferential temperature gradient obtained from the experiment and the simulation was 3.76°C/radian and 3.85°C/radian, respectively. Between the calandria tube and the moderator, the heat transfer coefficient was estimated to be around 822.3 W/m2‧K.

Radiative heat transfer analysis of 37-pin fuel bundle of Indian pressurized heavy water reactor under heat-up condition: experimental, numerical and analytical study

ABSTRACT Radiative heat transfer between the different components of the nuclear reactor channel (fuel pins, pressure tube (PT) and calandria tube (CT)) is the predominant mechanism of heat transfer in a channel when coolant flow is ceased. In this paper, the thermal behavior of the channel of Indian PHWR that has thirty-seven fuel elements is analyzed for a postulated accident such as loss of coolant accident (LOCA) coupled with the failure of the emergency core cooling system. (ECCS). The analysis has been performed using experimental, numerical and analytical techniques. The experiment is performed under the pseudo-steady-state condition, and the temperature profiles for the different parts of a simulated channel are obtained. The numerical analysis of the above problem is carried out using the ANSYS® Fluent 19.0. The code calculation is compared with the exact solution which is obtained using the radiosity network method. There is a reasonably good agreement between these three types of analysis. The results from these analyzes show that the temperature gradient (along the radial direction) is developed in the fuel bundle simulator (FBS) from the center fuel element to the outer ring fuel element. Also, an insignificant circumferential temperature gradient is observed in the FBS, PT and CT.

Analysis and evaluation of marine motor cooling system

Objective The design of the cooling system will directly affects the reliability of marine motors. It is necessary to evaluate the influence of the cooling systems on the marine motors. Method Taking a high-power-density marine motor as an example,, the overall model of the motor is established by using the electronic thermal simulation software ANSYS Icepak, and the three-dimensional temperature fields of three typical heat dissipation systems of spiral water jacket, axial water jacket and annular water jacket are calculated. The cooling effect of the components and temperature gradients are compared and evaluated. An optimization scheme based on the combination of forced air-cooling and water-cooling is proposed. Results The calculation results show that the water cooling system can solve the heat dissipation problem of the housing and stator, and reduce the axial and circumferential temperature gradients. The annular water jacket works best. However, the water-cooling system has a poor effect on the rotor and end housing. After the optimization, the rotor temperature and the overall temperature gradients of the motor are greatly reduced. Conclusion The annular water jacket is recommended to be chosen as the motor water cooling system, and the auxiliary cooling system should be considered to make up for the defects of the water cooling system.

Thermal-hydraulic-structural evaluation of S–CO2 cooling wall tubes: A thermal stress evaluating criterion and optimization

The S–CO2 Brayton cycle has been considered as a promising alternative in coal-fired power plants due to its high efficiency and high compactness. However, the high inlet temperature and low heat transfer coefficient of S–CO2 in the cooling wall tubes arranged in the furnace could lead to thermal fragile in practical operation. In present study, thermal-hydraulic-structural evaluation on S–CO2 cooling wall tubes is investigated. Firstly, it is found that the stress distribution is mainly dominated by the circumferential temperature gradient. Secondly, a new criterion, the thermal deviation factor (TDF), is proposed to evaluate the tube thermal stress. It indicates the plastic deformation if exceeding the critical value of TDF = 1, which is more efficient than conventional structural analysis. Thirdly, based on the TDF, two typical methods of inserting twisted tapes and unilaterally arranging dimples inside the tube are proposed to reduce the thermal stress. A novel configuration of elliptic dimple with rounded corners is finally proposed to reduce the local stress concentration efficiently. The results show that the elliptic dimples have the highest comprehensive performance, which is recommended as the heat transfer enhancement structures in S–CO2 cooling wall tubes.

Direct numerical simulation of the turbulent flow and heat transfer of supercritical CO2 in a semicircular pipe

The heat transfer characteristics of supercritical carbon dioxide (SCO2) in micro-channels is a major concern in the thermal-hydraulic design of printed circuit heat exchangers (PCHEs). In the present study, direct numerical simulation was utilized to study the heat transfer of SCO2 in a semicircular pipe under mixed and forced convection. The inlet pressure and temperature were set to 8 MPa and 301.15 K to study the effects of thermophysical property variations on heat transfer. The Reynolds number at the pipe inlet was 4000, which is comparable to that of a PCHE channel. Emphasis is placed on revealing the effects of heat flux, secondary flow, thermal acceleration, corner effect, and geometric orientation on heat transfer. It was found that the temperature at the top wall was always higher than that at the bottom wall owing to the effects of buoyancy. A high-temperature region was identified near the pipe corner (known as the corner effect), which is caused by the combined effects of reduced velocity and small local hydraulic diameter. Secondary flows with two large vortices were observed in the channel cross-section. The secondary flow intensity was strengthened by the increase in heat flux. At the same time, the vortex centers bent toward the bottom at high heat fluxes. The orientation of the semicircular pipe has a large effect on the cross-sectional vector distribution and circumferential temperature gradient, which should be considered in the design of PCHEs.

ASSESSMENT OF HEAT TRANSFER IN FUEL CHANNEL OF INDIAN PHWR UNDER POSTULATED LARGE BREAK LOSS OF COOLANT ACCIDENT: EXPERIMENTAL AND NUMERICAL STUDY

The behaviour of the channel under postulated large break LOCA scenario had been a prime safety concern. The radiative heat transfer is predominant in a channel when the convective cooling environment is marred. The estimation of temperature distribution in the fuel pins at elevated temperature is essential from the point of view of hydrogen gas generation and release of fission products. In this paper, the thermal characteristics of a channel for Indian PHWR under critical break failure is studied using experimental and numerical techniques. The experiment is carried out on an Indian PHWR having a fuel bundle of 37-fuel elements. The temperature profiles for different parts of the simulated channel comprising of fuel pins, PT and CT are obtained under steady condition. The numerical analysis is also performed using ANSYS Fluent 19.0. From the study, it is found that there is a significant radial temperature gradient in the fuel bundle from the center ring to the outer ring. Also, no significant circumferential temperature gradient is observed in the fuel bundle, PT and CT.

Experimental simulation of channel heat-up behaviour under slumped fuel pin condition for Indian PHWR

The thermal behavior of reactor channels is of prime concern for a postulated scenario of LOCA along with the un-availability of ECCS as the integrity of the fuel bundle may be challenged and slumping of fuel pins is expected. The present study is aimed to analyze the thermal characteristics of a large capacity Indian PHWR specific fuel bundle with disassembled fuel pins configuration. The configuration considers the collapse of 37-fuel pins of a bundle from the endplate and rearranges at the bottom surface of the PT. The paper further investigates the influence of PT-CT contact on the channel. The results show that a significant circumferential temperature gradient is developed in the PT due to the slumping of fuel pins. The maximum temperature is obtained at the bottom sector of the PT, which is in contact with the fuel pin. However, PT-CT contact causes a sudden temperature drop at the bottom nodes of the PT.

Numerical investigations on metal foam inserted solar parabolic trough DSG absorber tube for mitigating thermal gradients and enhancing heat transfer

Absorber tube in the superheating section of a direct steam generating solar parabolic trough collector field is subjected to large circumferential temperature gradients due to non-uniform concentration of solar flux and low thermal conductivity of steam. The thermal stresses developed due to the thermal gradients can lead to bending and may eventually result in the structural failure of the glass tube enclosing the absorber tube. Therefore, thermal analysis of the absorber tube for identifying the overheated sections and methods to mitigate the thermal gradients are very essential. In the present study, numerical investigations are carried to evaluate the performance of absorber tube with metal foam inserts subjected to non-uniform solar flux. Configurations of absorber tube with circumferential metal foam inserts in the lower half of the absorber tube are considered for the numerical analysis. Moreover, configurations of metal foam inserts with a novel shape based on the non-uniform flux around absorber tube are also analyzed. The metal foam inserted absorber tube configurations are found to be very effective in reducing the circumferential thermal gradients. In comparison to the clear tube, reduction of about 47% to 72% in maximum temperature difference is found for the range of operating conditions considered. The Nusselt number and performance evaluation criterion are found to be enhanced by a factor 10 and 5 respectively. Moreover, maximum gain of 3.71% in the net energy efficiency and 2.32% in exergy efficiency is found for the collector with metal foam inserted absorber tube configurations. The results of the numerical study conclude the insertion of metal foams as an effective method for mitigating thermal gradients around the tube and enhancing the heat transfer performance.

Influence of PT-CT contact on PHWR fuel channel thermal behaviour under accident condition – An experimental study

An experimental investigation has been performed to simulate a scenario of a beyond design basis accident like LOCA along with the un-availability of ECCS. The investigation is aimed to study the thermal behaviour of a single channel of PHWR during sagging deformation. The sagging deformation of PT was simulated by making a direct physical contact between PT and CT. The fuel channel assembly of a length of 1.4 m was immersed in water which simulates Moderator. The decay heat liberated after reactor shutdown was simulated by Joule heating of the 37-fuel pin elements. The steady state circumferential temperature distribution in the 37-fuel pin bundle simulator, PT and CT at different axial position has been obtained. It was found that there is a significant circumferential temperature gradient in the fuel channel. The maximum and minimum temperature was obtained respectively at top and bottom nodes of PT. However, an opposite trend was observed in CT. The maximum temperature difference of 185.7 °C and 10.2 °C has been found between top and bottom surfaces of PT and CT respectively. A similar behaviour to that of PT was discerned in the 37-pin fuel element. However, fuel pin of ring-1 has an insignificant effect of PT-CT contact. It was observed that around 84 percent of simulated decay heat is transferred to the simulated moderator.

CFD analysis of suspended debris during postulated severe core damage accident of PHWR

A steady state CFD analyses for suspended debris with two different configurations are carried out for heat transfer assessment. Very low frequency postulated accident without mitigation can lead to fuel channels exposure for Pressurized Heavy Water Reactor (PHWR). The exposed fuel channels constitute suspended debris. The debris configuration changes from regular to a disassembled configuration with accident progression. Each fuel channel consists of nineteen pin bundles, sagged Pressure Tube (PT) and Calandria Tube (CT). The study shows an asymmetric heat distribution from fuel bundle to surroundings for debris with disassembled channels. This is due to contact among the channels and among the components of an individual fuel channel. The asymmetric heat distribution leads to higher peak fuel pin, PT and CT temperatures as compared to debris with regular configuration. It also increases the circumferential temperature gradient in constituting components of a fuel channel. Radiative heat transfer from fuel channel surface constitutes the major heat transfer for both the debris configurations; the debris bed with disassembled channels radiates 71% of its total energy dissipation as compared to 65.3% for regular configuration. A good extent of convective heat transfer takes place for regular configuration as space among channels allows the steam flow to take place. The analyses shows that predicted temperatures of the debris bed is limited with convective and radiative cooling, thus a large scale of component oxidation is not anticipated. Radiation heat transfer is found to be the major mode of heat transfer. Hence, a numerical study of the effect of emissivity (0.3–0.8) variation on fuel channel heat up is also studied. The maximum temperature variation of 164.5 °C is observed at the bottom (0°) of CT with emissivity variation.

Role of substrate shape on thermal energy transmission in robotized wire and arc additive manufacturing

PurposeThe purpose of this study is to present how the thermal energy transmission of circular parts produced in robotized gas metal arc (GMA)-based additive manufacturing was affected by the substrate shape through finite element analysis, including distributions of thermal energy and temperature gradient in the molten pool and deposited layers.Design/methodology/approachThree geometric shapes, namely, square, rectangle and round were chosen in simulation, and validation tests were carried out by corresponding experiments.FindingsThe thermal energy conduction ability of the deposited layers is the best on the round substrate and the worst on the rectangular substrate. The axial maximum temperature gradients in the molten pool along the deposition path with the round substrate are the largest during the deposition process. At the deposition ending moment, the circumferential temperature gradients of all layers with the round substrate are the largest. A large temperature gradient usually stands for a good heat conduction condition. Altogether, the round substrate is more suitable for the fabrication of circular thin-walled parts.Originality/valueThe predicted thermal distributions of the circular thin-walled part with various substrate shapes are helpful to understand the influence of substrate shape on the thermal energy transmission behavior in GMA-based additive manufacturing.