Heat Transfer Characteristics Of Wall Research Articles

Experimental and numerical investigations on phase change thermal storage foam concrete based on CaCl2∙6H2O/silica aerogel composite phase change material

A new type of phase change thermal storage foam concrete was developed by effectively incorporating a composite phase change material (CPCM) into foam concrete. The CPCM was obtained by using 75 wt% CaCl2∙6H2O as PCM and 25 wt% silica aerogel as carrier, with a phase change temperature of 27.0 °C and an enthalpy value of 110.9 J/g. The mechanical and thermal properties of the obtained phase change thermal storage foam concrete blocks were investigated experimentally, along with a numerical analysis of their heat transfer characteristics. The experimental results demonstrated that after adding CPCM, the thermal performances of foam concrete blocks significantly increased, under the condition of their dry densities and compressive strengths met the specification requirements. Adding 20 wt% CPCM, the enthalpy value, specific heat capacity, thermal conductivity and thermal inertia of foam concrete block respectively reached 37.0 J/g, 1782.0 J/(kg∙K), 0.131 W/(m·K) and 770.9, with appropriate phase change temperature (27.2 °C). Numerical analysis for the heat transfer characteristics of the phase change thermal storage foam concrete wall revealed that optimal thermal insulation and thermal storage performances were achieved when the wall contained 20 wt% CPCM and thickness of 80 mm. In this case, the model exhibited a temperature wave attenuation factor of 6.66, the temperature amplitude of 5.84 K and a decline of 9.87 K for the highest temperature of the wall as well as delay time of 198,000 s, effectively regulating indoor temperature and saving energy. Therefore, the as-prepared phase change thermal storage foam concrete had great potential for application in building energy conservation.

Experimental and numerical study on the flow and heat transfer of ring-ribbed tank in deep-sea manned submersible

The ring-ribbed tank is the key equipment of cooling system of deep-sea manned submersible. The heat transfer characteristics of the outer wall and the heat transfer and flow characteristics of the inner flow channel of the ring-ribbed tank for different inlet and outlet temperatures and flow rates were studied through a water tank test. The results indicate that Nusselt number for the inner flow channel correlates with Reynolds and Prandtl numbers, while for the outer wall, it correlates with Grashof and Prandtl numbers with an overall error within 5%. A numerical study using the Realizable k-ε turbulence model and Boussinesq approximation of the flow-solid conjugate heat transfer is conducted, which can accurately determine the heat exchange capacity and the import and export differential pressure with an average relative error of 1.98% for hot water temperature and 7.73% for shell wall temperature. Additional simulations for determining the heat transfer capacity and flow resistance characteristics of the ring-ribbed tank in open water results demonstrate that the differential pressure and its coefficient are basically consistent with model test results. The cooling system can achieve a heat transfer rate of up to 17 kW with an external seawater flow velocity of approximately 2 knots.

Performance study of a compressed air energy storage system incorporating abandoned oil wells as air storage tank

With the rapid development of intermittent renewable energy, large-scale compressed air energy storage technology represented by Adiabatic Compressed Air Energy Storage (A-CAES) has attracted much attention. In order to simultaneously solve the problems of reuse of decommissioned oil wells and low efficiency of A-CAES system, a compressed air energy storage system incorporating abandoned oil wells as Air Storage Tank (AST) is proposed in this paper. The system performance of underground Oil Well CAES (OW-CAES), aboveground Steel Pipeline CAES (SP-CAES), and aboveground Storage Tank CAES (ST-CAES) is comparatively analyzed based on a thermodynamic model, focusing on the impact of heat transfer characteristic parameters of the AST wall on the system performance. The results show that recoverable heat and round-trip efficiency are significantly affected by the heat transfer characteristics of the AST wall. More recoverable heat and higher round-trip efficiency can be achieved by increasing the wall temperature and the surface area of the AST, respectively. The round-trip efficiency and energy storage density of the OW-CAES system are higher than those of the ST-CAES system, which are increased by 8.3 % and 18.45 % respectively. This study provides theoretical support for the feasibility of the OW-CAES system and has certain engineering guiding significance.

Investigation on heat transfer characteristics of multi-duct Trombe wall based on multi-mode heat exchange in winter

For the areas where solar energy resources are available and the demand is high, the low heat transfer efficiency of the traditional Trombe wall needs to be solved, so this study combines the traditional Trombe wall with tubular solar collectors and proposes a form of Trombe wall with multi-pipe and multi-heat transfer modes. Numerical simulation analysis is conducted based on the measured data near the wall surface of residential buildings in Zhengzhou City, China, in December. The thermal performance of different heat transfer modes is evaluated based on the simulation results, and the structural form of the multi-duct Trombe walls is optimized. The results show that the coupled heat transfer mode achieves the highest thermal efficiency. Compared with the traditional interlayer heat transfer mode, the coupled heat transfer mode has a faster start-up speed and more stable performance, with a 27.8 % increase in average daily thermal efficiency and a 36 % increase in heating capacity. Research has found that the 9-duct multi-duct Trombe wall performs better in terms of overall performance. Increasing the number of branch pipes further has limited effect on improving thermal performance. When the diameter of the heat collection branch pipe is 60 mm, the multi-duct Trombe wall achieves the highest average daily thermal efficiency of 70.3 %; adopting a non-uniform pipe distribution mode with the center distance of the middle 5 branch pipes being twice that of the other branch pipes, the Trombe wall achieves the highest average daily thermal efficiency and heating capacity.

Laminar flow and convective heat transfer characteristics of plain and wavy wall cross confined circular cylinder

An attempt is made to identify the convective heat transfer characteristics of the plain wall (Pw) and wavy wall (Ww) cross confined circular cylinder in the laminar flow regime (20 ≤ Re ≤ 100). The study has been extended to identify the wake heat transfer characteristics for various confinements (L = 0.5D, 1D, 1.5D and 2D), such that the weaker and stronger heat transfer region in the downstream of cross-confined cylinder has been investigated. CFD solver Ansys Fluent (Version 18.0) is used for assessing the field variables, the results are analyzed using streamline plots, λ 2-criterion vortex identification method, Nusselt number and field synergy method. The wall confinement is highly significant on heat transfer, the maximum Nusselt number is observed at high confinement (L = 0.5D) for all the Reynolds number studied. The changes in Nusselt number with respect to confinements is more significant at high confinements (L = 0.5D and L = 1D), become less significant at low confinements (L = 1.5D, 2D). It is found around 73% and 50% rise in Nusselt number for the confinement height, L = 0.5D as compared to L = 1D at Reynolds number 20 and100, respectively. Besides, in order to identify the heat stagnation region in the wake, contour of zero temperature gradient along the flow direction is observed. From the contour, it is identified that more heat stagnation region in the wake of Pw confined cylinder as compared to Ww confinement which is due to the three-dimensional behavior of flow in the wake.

가스터빈의 연소기 냉각을 위한 이중벽 냉각 구조에서 유출 구멍의 복합각도 변경에 따른 냉각 성능 변화에 대한 수치해석적 연구

For the cooling of the combustor liner, this investigation considers a double-wall cooling structure in which the compound angle and orientation angle of the effusion hole are 30° and ±30°. respectively. Also, to analyze the effect of orientation angle, data for comparison in a simple angle configuration with an orientation angle of 0° is provided. In this study, the heat transfer and flow characteristics of the double wall cooling structure by the change in orientation angle were numerically analyzed. The results of the numerical simulations was obtained using ANSYS FLUENT V 21.1 numerical codes with a shear stress transfer k-ω turbulence model. Data are provided for main flow Reynolds numbers from 142,000 to 149,000. and overall blowing ratios from 5.5 to 7.4. The use of compound and orientation angle arrangements inhibits the formation of horseshoe-shaped vortices,BRresulting in asymmetry of the counter rotating vortex. As a result, the lift-off of the coolant injected from the effusion holes is suppressed from the effusion cooling plate. When orientation angle is changed from 0° to ± 30°, the area-averaged film cooling effectiveness and area-averaged heat transfer coefficient increase by 7.7% and 12.4% overall, respectively. The skewed and non-symmetric shape of the horseshoe-shaped vortex is also increased by the use of orientation angle arrangement, which also provide localized improvements in surface thermal protection.

Multi-scale simulation study on the heat transfer characteristics of phase-change walls

The establishment of a multi-scale heat transfer model of the wall containing phase-change thermal insulation mortar (PCTIM) can clearly reflect the actual distribution of the phase change material in the PCTIM and the effect of the phase change components on the heat transfer characteristics of the wall. In this study, firstly, a mesoscopic two-phase stochastic composite model of PCTIM was established to calculate the effective thermal performance. The results showed that the thermal conductivity of PCTIM decreased with an increase of shaped-stabilized phase-change material (SSPCM) content. Then a multi-scale heat transfer model was used to study the influence of the content and the thermal property parameters of the phase-change material in PCTIM on the heat transfer characteristics of the outer wall. The results exhibited that the PCTIM with a phase-change temperature of 25–27 °C could play the role of increasing indoor thermal comfort in transition season and reducing building energy consumption in summer. In addition, with the increase of the latent heat and content of SSPCM, the temperature fluctuation of the phase change wall in the transition season was reduced by 31.1% and 30.6%, respectively, and the cumulative heat gain in summer was reduced by 8.8% and 16.8%, respectively, which effectively improved the temperature control effect of the phase-change wall in the transition season and the energy-saving effect in summer. However, with the increase of the thermal conductivity of SSPCM, the thermal regulation ability of the phase change wall in the transition season increased (the temperature fluctuation reduced by 6.7%), but the energy saving effect in summer decreased (the cumulative heat gain increased by 10.4%).

Numerical study on enhanced heat transfer and flow characteristics of supercritical hydrogen rocket engine's chamber wall using cylindrical ribs structure

One of the most critical issues in supersonic flight is the thermal protection of the rocket engine combustion chamber. The ribs' construction can improve heat transfer and coolant flow characteristics significantly. Ansys fluent was used to investigate the supercritical hydrogen fuel flow and heat transfer characteristics. Ribs spacing of 10 mm, 5 mm, and 2.5 mm were compared to determine the optimal spacing required to achieve maximum cooling influence and lower pressure drop to safeguard the overheated structure and lower the temperature of the walls. Compared to the smooth channel, the thermal performance factor and pressure drop of the 5 mm spacing increased by 32.5% and 16.64%, respectively, while the maximum temperature reduced by 12.07%. Therefore, the cylindrical rib construction with a 5 mm spacing is perfect for improving heat transfer, has acceptable pressure drops, and reduces thermal stratification inside the channel.

Flow and heat transfer characteristics of seepage-channel wall to injected suspended particles in a groundwater heat pump system

Well and aquifer clogging caused by deposition of particles is a serious problem that restricts the application and heat transfer efficiency of groundwater heat-pump systems. Mixed flow of particles and fluid in a porous medium was simplified to pipe flow based on a capillary model. A migration test bench for suspended particles was set up to explore the flow and heat transfer response of the seepage channel to the injecting suspension by varying suspended particle size (Dp), roughness of pipe wall (K), and injection pressure (Pin). The results show that the pressure drop of the suspension was influenced by the factors in the order Pin > Dp > K. Suspended particles were deposited at the entrance of the seepage channel, and deposition increased with increasing particle size. The temperature variation of the channel wall was sensitive to particle size. The temperature gradient of the suspension with a particle median size of 17.62 μm was 0.049 °C/min when the injection pressure was 1.8 kPa and wall roughness was 1.65 mm, which was lower than at 0.148 °C/min, when the particle median size was 25.12 μm. This research provides insights into the design and optimization of injection projects to prevent particle clogging.

Effect of heat and moisture transfer on the growth of mould on the inner surface of walls: A case study in Dalian of China

Due to the influence of outdoor climate and indoor human activities, the moisture accumulation of building enclosure structure will lead to the growth of mould on the interior surface of the wall, damage the building structure and endanger human health. Taking an office building in Dachangshan Island of Dalian, a cold area in North China, as an example, the outdoor relative humidity is high, and the temperature difference between indoor and outdoor is large during the heating period, which is easy to cause the increase of moisture content of the enclosure structure and the growth of mould. This study focuses on the influence of the heat and moisture transfer characteristics of the office building wall on the growth process of mould on the interior surface of the building. The dominant fungi of indoor air and wall surface were Penicillium and Cladosporium. WUFI software was used to simulate the heat and moisture transfer and the growth process of mould on the inner surface of the wall. The results show that the relative humidity of the air micro layer on the inner wall surface of the office building reaches more than 80% at the beginning of heating period in November every year, the mould hypha grows significantly in December and brings pollution risk. Finally, the relative humidity drops below 70% after the heating period is over in April, and the mould stops growing. The main reason for this phenomenon is that the condensation interface in the wall structure during the heating period leads to the accumulation of water in the wall, which is then transmitted to the wall surface through the wall pores, thus increasing the moisture content. This method provides a r eliable way for the numerical simulation of indoor mould growth risk.

Flow and heat transfer measurements in the laminar wake region of semi-circular cylinder embedded within a rectangular channel

Flow and heat transfer in the wake region of a semi-circular cylinder embedded within a rectangular channel have been studied experimentally using non-intrusive techniques. The semi-circular cylinder embedded within the channel with a blockage ratio of 0.45 (d/H) acts as an obstacle to the incoming fully developed laminar flow and creates a wake region in its downstream side. The velocity field in the downstream wake region is mapped using PIV (particle image velocimetry) technique with water as the working medium. The fluctuating shear layers and the vortices shed by the cylinder interact with the bottom heated wall of the channel. The influence of the wake region of the cylinder manifests on the heat transfer characteristics of the bottom wall of the channel and acts in a way to passively improve the overall heat transfer characteristics. The resulting whole field temperature values are captured using interferometry and augmentation in heat transfer rates vis-à-vis a channel without cylinder are compared. The dominant frequencies of the downstream unsteady flow in the wake of the cylinder are captured using schlieren technique. Heat transfer enhancement up to 42 percentage is observed in the case of the presence of the semi-circular cylinder in the channel in comparison with the flow in a plain channel for the Reynolds number range of 75–200 under which the experiments are performed.

Heat transfer characteristics of a composite radiant wall under cooling/heating conditions

The radiant wall composited with capillary tubes has been widely applied in heating or cooling systems due to its large heat transfer area, low-temperature heating and high-temperature cooling. In this study, a ratio model of heat transfer in steady-state condition was established, which explores heat transfer capacity from the capillary layer (active layer) towards the indoor and outdoor sides. The experimental data including the radiant surface temperature, the capillary layer temperature and the heat flux distribution were collected in cooling and heating conditions. The proposed ratio model was validated. The results show that the fluctuation of indoor air temperature is relatively small, suggesting that the radiant system possesses higher stability. Results showed that thermal resistances of the composite radiant wall in summer and winter conditions vary greatly due to different moisture contents. With the continuation of the system operation, the calculated values from the ratio model under the steady-state condition were more consistent with average values obtained from experiments under unsteady-state conditions, indicating that the overall heat transfer performance of the composite radiant wall could be properly evaluated by the proposed model in engineering applications.

Thermal Analysis of Paper Board Packaging with Phase Change Material: A Numerical Study

Phase change materials (PCM) absorb, store, and release thermal energy by changing phase, usually between solid and liquid, to maintain a regulated temperature. Compared with conventional sensible heat storage materials, PCMs can absorb or release large amount of heat without a significant change in temperature until the phase change process is complete. There are many potential applications of PCMs, including energy storage system, electronic/battery cooling, thermal management in buildings, and temperature controlled packaging. This study fundamentally explores the effect of the presence of PCMs on the heat transfer characteristics of packaging wall. The presence of PCM is studied in two different configurations: a layer of PCM embedded in the center of a paper board, and uniformly distributed PCM particles within a paper board. The numerical results presented here are based on a transient conjugate heat transfer analysis with natural convection present on the both sides of the packaging wall. A parametric study is performed on the PCM layer thickness to determine the impact of PCM on isolating the package interior temperature from ambient conditions over an extended time. The numerical results show significant reduction in the transfer of heat from the exterior to the interior of the packaging wall in the presence of PCM. This extends the time for the interior temperature to equalize with the exterior temperature under the operating conditions considered in this study.

Investigation of Geometry Effect on Heat and Mass Transfer in Buoyancy Assisting with the Vertical Backward and Forward Facing Steps

Abstract This paper is a Direct Numerical Simulation (DNS) of the temperature distribution in the flow with the forward and backward facing steps with buoyancy forces for different lengths of the bottom walls and flow regimes. The different lengths of the bottom walls effect were studied with buoyancy forces. A two-dimensional incompressible Navier–Stokes equation and equations for temperature transfer were used to describe this process. The obtained numerical solutions for the test problem, laminar flow with the backward facing step with buoyancy force, are compared with the numerical results of other authors. A verified numerical algorithm is applied to the problem with the forward and backward facing steps with buoyancy forces. The influence of changes in the bottom wall length on temperature distributions and velocity components is revealed. It was also revealed that the heat transfer characteristics of the bottom wall have strong changes in different lengths. In cases for the lengths of the bottom walls ( X e = 12 , 16 ${X_e} = 12, \,16$ ) at Re=1,000, the rotational movement of the vortices along the channel wall revealed an increase in the exchange of cold and hot fluids, which leads to temperature fluctuations from the lower wall to the upper wall of the channel.

A dual zone thermodynamic model for refueling hydrogen vehicles

With the simple structure and quick refueling process, the compressed hydrogen storage system is currently widely used. However, thermal effects during charging-discharging cycle may induce temperature change in storage tank, which has significant impact on the performance of hydrogen storage and the safety of hydrogen storage tank. To address this issue, we once propose a single zone lumped parameter model to obtain the analytical solution of hydrogen temperature and use the analytical solution to estimate the hydrogen temperature, but the effect of the tank wall is ignored. For better description of the heat transfer characteristics of the tank wall, a dual zone (hydrogen gas and tank wall) lumped parameter model will be considered for widely representation of the reference (experimental or simulated) data. Now, we extend the single zone model to the dual zone model which uses two different temperatures for gas zone and wall zone. The dual zone model contains two coupled differential equations. To solve them and obtain the solution, we use the method of decoupling the coupled differential equations and coupling the solutions of the decoupled differential equations. The steps of the method include: (1) Decoupling of coupled differential equations; (2) Solving decoupled differential equations; (3) Coupling of solutions of differential equations; (4) Solving coupled algebraic equations. Herein, three cases are taken into consideration: constant inflow/outflow temperature, variable inflow/outflow temperature and constant inflow temperature and variable outflow temperature. The corresponding approximate analytical solutions of hydrogen temperature and wall temperature can be obtained. The hydrogen pressure can be calculated from the hydrogen temperature and the hydrogen mass using the equation of state for ideal gas. Besides, the two coupled differential equations can also be solved numerically and the simulated solution can also be obtained. This study will help to set up a formula based approach of refueling protocol for gaseous hydrogen vehicles.

Dynamic heat transfer characteristics of wall implanted with heat pipes in summer

The heat transfer characteristics of a wall implanted with heat pipes, a new passive natural energy utilization technology, significantly differ from that of an ordinary wall. A dynamic heat transfer model is established in this study via steady heat transfer experiment and theoretical analysis for the north wall implanted with heat pipes in summer months; relevant heat transfer characteristics are studied through dynamic simulation analysis. Results show that the inner temperature field of the north WIHP presents obvious upper and lower partitions in summer. The thermal response of the WIHP is faster than that of an ordinary wall. The delay time of the ordinary wall is higher than the WIHP by 1.38 h. The average temperature of the north WIHP is 2 °C lower than that of an ordinary wall, which represents an effective improvement to the indoor thermal environment. The accumulative heat transfer capacity of the north WIHP is relatively high, at 50.7 kW/m2

Heat transfer characteristics of spiral water wall tube in a 1000 MW ultra-supercritical boiler with wide operating load mode

Nowadays, heavy peak-shaving task requires ultra-supercritical boilers having fast and flexible response-characteristic. Wide operating loads and quick load-change mode ask for better flow and heat transfer characteristics of the water wall. Under the above backgrounds, heat transfer to water flowing upward in an inclined smooth bare tube was experimentally investigated under the operating conditions of an ultra-supercritical boiler. The experimental results show that the metal temperature of the tested water wall tube is safe from 30% to 90% BMCR. The heat transfer coefficient of single-phase water at bottom side is 1–2 times bigger than that at the top. The Fr number put forward by Kefer et al. can give a convincing prediction on the effect of flow stratification in the tested pressure range 12–18 MPa. The calculated steam quality difference with the Kefer et al. correlation is an average-value prediction on axial length of the transitional region. At supercritical pressures, four widely used heat transfer correlations are evaluated with the experimental data. The comparison results demonstrate that Swenson et al. correlation gives the best prediction on heat transfer. The effect of flow stratification is evaluated by the modified correlation of Petukhov and Popov, which demonstrates that the effect of flow stratification can be neglected in the high enthalpy region.

Numerical Simulation of a Radiant Floor Cooling Office Based on CFD-BES Coupling and FEM

In this paper, in order to accurately simulate building thermal performance, a coupling strategy between the hourly Computational Fluid Dynamics (CFD) simulation of the indoor airflow in a room and the dynamic Building Energy Simulation (BES) of the heat transfer through its envelope based on real-time information exchange is developed, and the radiation heat transfer among the internal surfaces of its envelope is also calculated. The governing equations are solved by Finite Element Method (FEM). Then, the developed program is applied to co-simulate an office room with a ventilating and radiant floor cooling system. The results show that the dynamic heat transfer characteristics of the external wall and the indoor air in the room change with outside weather conditions in detail.

Wall inclination effect in heat transfer characteristics of a combined wall and offset jet flow

In the present work, a numerical study of the heat transfer in an oblique combined turbulent wall and offset jet flow was performed. Different turbulence models were tested: the standard k−ω, SST k−ω, standard k−ɛ, RNG k−ɛ and realizable k−ɛ. A parametric study was also carried out to investigate the wall inclination β and the Reynolds number Re effect on the local Nusselt number Nu, local wall heat flux q, average Nusselt number Nu¯ and average wall heat flux q¯. Correlations witch link the local and the average Nusselt number and wall heat flux to the wall inclination β and the Reynolds number Re were also provided in this paper. Constant wall heat flux and constant wall temperature boundary conditions were considered in all computation. Analyzing the numerical results show that the heat transfer exchanged between the oblique wall and the whole flow is more intense for low values of wall inclinations β, for high values of Reynolds numbers Re and under constant wall heat flux boundary condition.

Study on heat transfer characteristics of straw block wall in solar greenhouse

A new type of solar greenhouse with straw block north wall was developed. The heat transfer characteristics, temperature, heat absorption, release and loss of its north wall were investigated by theoretical and experimental analyses and the economic and environmental evaluations of north walls were carried out. The thermal storage coefficient of heat storage layer, the total thermal inertia index and the total thermal resistance of the new type straw block north wall (ST) were 38.5%, 13.9% and 25.4% higher than that of porous clay block north wall (CL), respectively and the average air temperature of Chinese solar greenhouse with ST was 0.7°C higher than that of Chinese solar greenhouse with CL on cloudy day. The temperature of heat storage layer as well as the heat absorption and release of ST were higher than that of CL. The heat loss of ST was less than that of CL. Compared to CL, the cost, energy consumption and CO2 emissions of ST were reduced 26.3%, 81.7% and 29.4%, respectively. ST had better thermal properties and could improve thermal environment of greenhouse. Besides, it was low-cost and environmentally friendly.