Local Convective Heat Transfer Research Articles

Analysis of the convective heat transfer and equivalent thermal conductivity of functional paper honeycomb wall plates

ABSTRACT For the development of functional honeycomb plates (FHPs) and efficient resource utilization of straw, based on the measurement of the equivalent thermal conductivity (λE) of paper honeycomb plates (HPs) and FHPs, including straw-core paper honeycomb plates (SHPs), under cold-left and hot-right conditions, the influence mechanism of the system geometry parameters on the convective heat transfer of HPs and the heat insulation performance of FHPs by fillers are discussed. The results are as follows. 1) Compared with the case of hot-above and cold-below conditions, the convective heat transfer ratio of HP increases substantially under cold-left and hot-right conditions. 2) The main heat transfer modes of the FHPs are radiative heat transfer between the wall (including the panel and side) and filling material and solid heat conduction in the filling material, supplemented by a small amount of heat conduction through air or local convection heat transfer in the void. 3) The higher the temperature is, the greater the FHP equivalent thermal conductivity, with little influence from the heat transfer direction. 4) It is relatively conservative to use the heat transfer effect of HPs and FHPs to evaluate that of beetle elytron plates (BEPs) and functional beetle elytron plates (FBEPs). This study lays a foundation for the application of multifunctional BEPs and HPs in building envelopes.

Numerical analysis on thermo-fluid–structural performance of graded lattice channels produced by metal additive manufacturing

• Thermo-fluid–structural performances of graded lattice channels are compared. • Increase-type graded lattice channel exhibits the best temperature uniformity. • V-type graded lattice channel exhibits the worst thermo-fluid performance. • W-type graded lattice channel exhibits the best thermo-fluid–structural performance. The graded lattice channel is a novel design for improving heat transfer and structural stability by effectively varying the volume fraction for various applications. However, the combined analysis of the thermo-fluid and structural performances of the graded lattice channel is very limited in the literature. In this study, the thermo-fluid–structural performances of the increase-type graded (IG), V-type graded (VG), and W-type graded (WG) lattice channels were investigated using a thermo-fluid–structural interaction one-way coupled model and compared with that of a uniform lattice channel. The results indicated that the increase-type graded lattice channel had the lowest standard deviation of the working surface temperature owing to an increase in its local convective heat transfer. The V-type lattice channel exhibited the lowest thermo-fluid performance owing to the highest difference in the volume fraction between unit cells. The W-type graded lattice channel exhibited the lowest maximum stress because of its highest support structure. Furthermore, under various inlet velocity and heat flux conditions, the W-type graded lattice channel exhibited superior thermo-fluid–structural performance owing to its high thermo-fluid performance and low stress ratio, when compared with those of other lattice channels. Overall, the graded lattice channels can be recommended as a cooling channel of high-performance electronic devices and manufacturing tools.

Effects of Jet Impingement on Convective Heat Transfer in Effusion Holes

Abstract A common design for cooling the combustor liner of gas turbines is a double wall composed of impingement jets that feed effusion cooling holes. An important cooling mechanism associated with the effusion hole is the convective cooling provided to the liner wall, which is in contact with the hot main gas flowing through the combustor. While the combination of impingement jets and effusion holes has been studied earlier, mostly in terms of cooling effectiveness, investigators have not fully evaluated the effect the impingement jet has on the local internal convection within the effusion hole. This study evaluates the detailed effects of the impingement geometry on the local convective heat transfer coefficients within the effusion hole, which provides insights as to the design decisions for cooling combustor liners. Using a scaled-up, 3D-printed effusion hole with a constant heat flux boundary condition, local convective heat transfer coefficients were measured for a range of impingement geometries and positions relative to the effusion holes. Results showed a strong influence on the convective heat transfer resulting from the placement of the impingement hole relative to the effusion hole. In particular, the results showed a strong sensitivity to the circumferential and radial placement of the impingement jet with little sensitivity to the jet-to-effusion distance.

Effects of piezoelectric fan on cooling flat plate in quiescent air

A test facility was built to investigate the cooling effects of a piezoelectric fan on a heated flat plate in a quiescent atmosphere. The facility consists of a heat source of a thin Ni-Cr foil and a piezoelectric fan for cooling the foil, where the surface temperature on the foil was measured by an IR camera. 3D unsteady numerical simulations were then conducted to analyze the relationship between the jet flow from the piezoelectric fan and the temperature distribution on the heated plate. The measured temperature data was used for validation of the numerical results. Without the fan, the temperature is highest at the center of the plate and lowers toward the plate edge. With the operation of the fan, the temperature at the center of the plate is locally reduced by 28 °C. Based on the experiments and simulations, it is confirmed that the piezoelectric fan is very effective to cool the plate locally. In addition, it was found that the local convective heat transfer coefficient is closely related to the movement of the vortices generated by the fan.

Convection heat transfer of supercritical CO2 in a single fracture in enhanced geothermal systems

EGS (Enhanced Geothermal System) is a potential way to utilize geothermal energy efficiently. In present work, a three-dimensional physical model, a single fracture with undulated surface morphology in hot dry rock is constructed. Supercritical CO2 is used as working fluid, and its convection heat transfer characteristics in fracture is numerically investigated. The influence of flow rate, inlet temperature, and reservoir pressure on convection heat transfer are probed. The results indicate that the local convection heat transfer coefficients at upper wall and lower wall of fracture show opposite variation trend at corresponding positions, and it varies with the undulation of surface morphology. In addition, the local convection heat transfer coefficients rise with the decrease of flow rate, the increase of pressure and inlet temperature. Furthermore, the correlation of normalized convection coefficient Nusselt number of supercritical CO2 in fracture is obtained.

Double‐diffusive natural convective stream due to moving vertical plate with nonlinear thermal radiation and Newton's boundary constraint

AbstractThis article investigates the heat and mass transmission of the double‐diffusive convective stream over a moving vertical plate with nonlinear thermal radiation and newton boundary conditions. The governing partial differential equations of the stream, heat, and concentration profiles were transformed into a system of nonlinear ordinary differential equation by utilizing resemblance transformation. This system was then resolved numerically by applying the fourth order Runge‐Kutta method with most efficient shooting technique. The effect of convection, buoyancy ratio, nonlinear thermal radiation, Prandtl number, Rayleigh number and Schmidt number are graphically scrutinized. The numerical results are obtained for velocity, temperature, and concentration profiles. It is found that when the velocity profile increases, heat and mass transfer rate decreases with an increase in the parametric value of buoyancy ratio parameter. It is found that the effect of nonlinear thermal radiation stabilizes the thermal boundary layer growth. The skin friction coefficient decreases with an increase in Prandtl number. However, the Nusselt number increases with an increase in the local convective heat transfer rate. The present results are very much promising, and further, there is a very good agreement of results when compared with earlier published results for some limiting conditions.

Numerical and Analytical Investigation of Heat Transfer Mechanisms and Flow Phenomena in an Intermediate Pressure Steam Turbine Blading During Startup

Abstract As a result of an ever-increasing share of volatile renewable energies on the worldwide power generation, conventional thermal power plants face high technical challenges in terms of operational flexibility. Consequently, the number of startups and shutdowns grows, causing high thermal stresses in the thick-walled components and thus reduces lifetime and increases product costs. To fulfill the lifetime requirements, an accurate prediction and determination of the metal temperature distribution inside these components is crucial. Therefore, boundary conditions in terms of local fluid temperatures as well as heat transfer coefficients (HTCs) with sufficient accuracy are required. As modern numerical modeling approaches, like 3D-conjugate-heat-transfer (CHT), provide these thermal conditions with a huge calculation expense for multistage turbines, simplified methods are inevitable. Analytical heat transfer correlations are thus the state-of-the-art approach to capture the heat transport phenomena and to optimize and design high efficient startup curves for flexible power market. The objective of this paper is to understand the predominant basic heat transfer mechanisms such as conduction, convection, and radiation during a startup of an intermediate pressure (IP) steam turbine stage. Convective heat transport is described by means of heat transfer coefficients as a function of the most relevant dimensionless, aero-thermal operating parameters, considering predominant flow structures. Based on steady-state and transient CHT simulations, the heat transfer coefficients are derived during startup procedure and compared to analytical correlations from the literature, which allow the calculation of the heat exchange for a whole multistage in an economic and timesaving way. The simulations point out that the local convective heat transfer coefficient generally increases with increasing axial and circumferential Reynolds' number and is mostly influenced by vortex systems such as passage and horseshoe vortices. The heat transfer coefficients at vane, blade, hub, and labyrinth-sealing surfaces can be modeled with a high accuracy using a linear relation with respect to the total Reynolds' number. The comparison illustrates that the analytical correlations underestimate the convective heat transfer by approximately 40% on average. Results show that special correlation-based approaches from the literature are a particularly suitable and efficient procedure to predict the heat transfer within steam turbines in the thermal design process. Overall, the computational effort can be significantly reduced by applying analytical correlations while maintaining a satisfactory accuracy.

Experimental and numerical study of flow and thermal transport in fractured rock

An experimental and numerical study was carried out to investigate matrix-fracture thermal transport in fractured rock. Two different synthetically fractured core plugs were used during the flow-through experiments while core plugs’ outer surface was maintained at two different constant temperatures. To investigate the matrix-fracture thermal transport, cold water was injected through the single fracture core plugs at different flow rates. A film type heat flux sensor was used in the fractured core plug to measure the fracture temperature and heat flux over the matrix-fracture interface. Experimental results showed that fracture temperature decreases with increasing flow rate while heat flux increases. At high flow rates, temperature difference over matrix-fracture interface increases due to large volume of water contacting the fracture at a given time. Water temperature decreased more in the case of fracture surrounded by rock matrix with low volumetric heat capacity and thermal conductivity. A numerical model calibrated using experimental values of the fracture temperature was developed to estimate thermal properties of rock matrix and to identify their importance on the matrix-fracture thermal transport. Using the calibrated numerical model, it has been shown that temperature in the fracture and fracture surface temperature are different contrary to previous studies assuming the same values for these two temperatures. Local and transient convective heat transfer coefficients were introduced with respect to local values of temperature difference and transient values of heat flux along matrix- fracture interface enabling accurate representation of matrix-fracture thermal transport.

Experimental investigation of convective heat transfer using ethylene glycol-based nano-fluid

Coolant plays important characteristic in automobile industry to prevent failure and damage by balancing the temperature. Due to this approach, coolants are being used as new thermal fluid to study the heat transfer coefficient performance. This study consists of an experimental investigation of internal convective heat transfer of 50:50 Water-Ethylene Glycol based Nano-fluid through a copper tube of 18mm external diameter and 16.5mm internal diameter and a test section of 1m in a fully turbulent regime. Total convective heat transfer coefficient of Nano fluid at three different volumetric concentrations of nanoparticles is estimated. Local convective heat transfer at eight different points along the tube at varying Reynolds number is also determined. At 0.15% volumetric concentration of SiO2 Nanoparticles (NPS) 29% increment in convective heat transfer coefficient (CHT) is observed. The decrease in the heat transfer rate is observed with changing distance axially. Particles disorganized movement of NPs and undulation in the fluid and increased in thermal conductivity of Nano fluid can be possible reason for extra ordinary change in heat transfer.

Convective heat transfer characteristics of supercritical carbon dioxide in vertical miniature tubes of a uniform heating experimental system

This study experimentally investigates the local convective heat transfer characteristics of supercritical carbon dioxide in vertical and uniformly-heated miniature tubes with inner diameters of 0.5, 0.75, and 1.0 mm, and compares with that for horizontal flow. The effects of outlet pressure, heat flux, mass flux, inlet temperature, and tube diameter on the local heat transfer are explored through the evaluation of local heat transfer coefficient and local wall temperature. The results reveal that the non-heat-transfer-deterioration (non-HTD) mode is prevalent for upward flow in the vertical miniature tubes because of the effects of buoyancy and flow acceleration. The optimal heat flux for the best heat transfer performance occurs when the outlet fluid temperature is close to the corresponding pseudocritical point. Moreover, the local heat transfer coefficient increases with an increase in the mass flux but decreases with an increase in the inlet fluid temperature or tube diameter. An increase in the outlet pressure causes increases in the fluid and wall temperatures. The experimental results show that the buoyancy effect would promote the local heat transfer for vertical downward flow but deteriorate that for vertical upward flow, particularly after the pseudo-critical region. The horizontal flow would present the best heat transfer performance when the outlet flow condition is near the pseudo-critical point, while the vertical downward flow is superior for the system with the outlet fluid temperature substantially higher than the pseudo-critical point. The new empirical correlation developed can predict very well the present data set of local Nusselt number for upward flow in vertical uniformly-heated tubes within a relative error of ±20%.

Numerical study of the convective heat transfer coefficient of the hand and the effect of wind

This study aimed to investigate the effect of wind speed and direction on the convective heat transfer coefficient of the human hand, considering the specific geometry of human hands. Computational fluid dynamics (CFD) simulation of the heat transfer and airflow between the hand and surrounding environment was performed. The numerical hand form was created from a surface scan of a thermal hand manikin and divided into 18 sections, including the palm, dorsal, wrist, and five fingers, with each section divided into three parts: fingertip, middle digit, and finger root. The numerical simulation results were compared with the experiment data and showed good conformity. The numerical results showed that the local convective heat transfer coefficient of each of the 18 hand sections varied significantly due to the specific local geometry of the hand. The convective heat transfer coefficients of the five fingers are larger than those of the palm, dorsal, and wrist, and the fingertip is the most vulnerable section to heat loss. The convective heat transfer coefficient of the whole hand increased from 4.30 to 26.49 W/(m2·K) when wind speeds increased from 0.05 to 2.0 m/s. Wind directions had little effect on the convective heat transfer coefficient of the whole hand but had a significant impact on different individual hand sections. The CFD model, along with the developed regression equations of convective heat transfer coefficient with wind speed, can be a foundation for further study on the interaction of human hands with various thermal environments to improve thermal comfort and work efficiency.

Heat Transfer in Horizontal Copper Tube Heated by Electric Heating Process

Heat transfer from electrical and electronics component is essential for better performance of that electrical system, The maximum heat transfer from that system results long period durability. In most of the system base provided for equipments are very small and placed in a very complicated position. so heat transfer by forced convection is not easy for that purpose.The heat transfer by natural convection is the familiar technique used in electronics cooling; there is huge group of apparatus that lends itself to natural convection .This category consist of stand-alone correspondence such as modems and small computers having an array of printed circuit boards (PCB) accumulate within an area. Natural convection heat transfer in heated horizontal duct drive away heat from the interior surface is offered. The duct is open-ended and round in cross section. The test section is heated by provision of heating coils, where constant wall heat flux mentioned. Heat transfer experiment is carried out for channel of 50 mm. internal diameter and 4 mm thickness with length 600 mm. Ratios of length to diameter of the channel are taken as L/D = 12. Wall heat fluxes maintained at q// = 300 W/m2 to 3150 W/m2 . A methodical investigational record for the localsteady state natural convection heat transfer activities is obtained. The wall heating condition on local steady-state heat transfer phenomena are studied.The present experimental data is compared with the existing theoretical and experimental results for the cases of vertical smooth tubes.

Tomography-based investigation of flow and heat transfer inside reticulated porous ceramics

This study investigates the flow and heat transfer within reticulated porous ceramics, with the aim of enhancing the understanding of phenomena occurring at the pore scale in order to extract useful information for the optimisation of heat sinks or other reticulated porous devices in engineering applications. In this study, X-ray computed tomography is used to reconstruct the reticulated porous ceramic. From the voxel mesh, the real geometry of the foam sample can be obtained, and an unstructured mesh is generated using ICEM. Then, a fully coupled heat transfer model is established. Based on this model, the transient conjugate heat transfer between the airstream and the porous matrix is solved using FLUENT software. The numerical results show that the pressure drop vs. the superficial velocity follows the Darcy–Forchheimer equation, and the border-affected region is only 1–2 times the mean cell size. Moreover, a local thermal equilibrium lasting for 7–10 s is found in the transient coupled flow and heat transfer process in this study. Additionally, the local convective heat transfer coefficient, which mainly depends on the velocity, is constant along the flow direction, and the instantaneous volumetric heat transfer coefficient increases monotonically with time. The obtained results are important theoretical supplements for understanding the flow and heat transfer in porous materials, and are crucial for enhancing the heat transfer in applications of reticulated porous ceramic materials.

A Similarity Solution for Natural Convection Flow near a Vertical Plate with Thermal Radiation

This paper examined the fluid transport on boundary layer near a vertical wall with thermal radiation effect. The coupled nonlinear partial differential equations are reduced to a system of ordinary differential equations through similarity transformation. The resultant system of ordinary differential equations is integrated with maple software using RKF45 method. The effect of the active parameters such as Grashof number, thermal radiation parameter, temperature difference, Prandtl number, and local convective heat transfer parameter on the velocity profile, skin friction, temperature profile and Nusselt number are presented using graphs and tables. The outcome of the study revealed that the Prandtl number has a decreasing effect on thermal boundary layer thickness. It also revealed that the thermal radiation has an increasing effect on the Nusselt number, temperature distribution, temperature gradient and skin friction. The temperature distribution increases with increasing values of the temperature difference and local convective heat transfer parameter whereas decreases with Grashof number.

Numerical simulations of airflow and convective heat transfer of a sow

Effective ventilation reduces heat stress in sows. A numerical study was carried out to investigate the effects of airspeed and direction on the convective heat transfer around a sow having different postures and body mass. The convection characteristics of a sow were analysed in a virtual wind tunnel using computational fluid dynamics (CFD) tools. The four postures of the standing, sitting, reclining and lateral lying were included. The investigated sow masses ranged from 130 kg to 340 kg. The results show that: 1) The general convective heat transfer was higher when the sow was standing or sitting postures rather than in a reclining or lateral lying posture; 2) Wind direction affects the sow convective heat transfer coefficient. The convective heat transfer coefficient of a sow was largest when it was standing or sitting with the body axis 60° to wind direction. When reclining or lateral lying sow axis at 45° to wind direction, the convective heat transfer coefficient was the largest; 3) The convective heat transfer coefficient of a single sow can be used to predict the convective heat transfer coefficient of a sow amidst rows of multiple sows; 4) Sow body mass negatively affects the convective heat transfer coefficient; 5) The convective heat transfer coefficient of the sow's trunk is relatively low compared with other parts such as the legs, head etc. Based on these results, when a sow needs to be cooled during hot conditions, an oblique airflow is recommended, and airflow speed around the sow's trunk should be increased as much as possible. • Lowest sow heat transfer coefficient in reclining posture compared to others. • Airspeed and direction affected sow convective heat transfer coefficient. • Local convective heat transfer coefficient on trunk lowest compared to others. • Equations for estimating sow's convective heat transfer coefficient established.

Infrared image processing for local convective heat transfer measurements in rib-enhanced channels

In the context of an experimental study designed to retrieve the local heat transfer characteristics in closed channels with ribs in several configurations by IR thermography, a filtering strategy must be adopted to calculate the laplacian of the temperature field, to remove not only Gaussian noise, but also unwanted local features due to the pattern of the conductors of the heater. In this paper, these issues are addressed with a comparative analysis on filtering and interpolation techniques, in particular mean filtering, local surface fitting and cubic smoothing splines, which are carried out by showing their influence on the smoothed thermogram and on the laplacian of the temperature field. The use of cubic smoothing splines gives the best approximation of the temperature field with respect to the mean filter, which alters the temperature field in proximity of the ribs, and to surface interpolation. However, the only filter among those tried that gives meaningful derivatives on the whole image is the mean filter, with a kernel size equal or bigger to that of the spatial feature to be filtered. The resulting derivative also need further filtering to reduce local spikes induced by the filtering operations.

Conjugate numerical analysis on bundle effects for integrated heat transfer of LNG ambient air vaporizer

Ambient air vaporizer (AAV) is widely used to vaporize liquefied natural gas (LNG) in gas terminal stations due to its low cost in operating and eco-friendly advantages. Its vaporization efficiency depends on the overall heat transfer performance of the finned tube bundle, which was mostly represented by single tube in previous researches. This paper presented a numerical investigation on the integrated heat transfer of AAV tube bundle considering the heat transfer conjugate problem between different zones. An experimental test combined with the nondimensional correlation in finite space was conducted to validate the numerical model, and the results are in good agreement. The results indicate that the surrounding tubes weakens the field synergy of the flow and heat transfer in inner area of the AAV array, resulting in the decline of heat transfer coefficient from far to near the center of the array. For each finned tube in the AAV bundle, the circumferential local convective heat transfer coefficients decrease from 45° to 225° angular space and then arise from 225° to 360° angular space. Besides, poorer convective heat transfer of the air will lead to the delay of LNG multiphase flow with higher heat transfer intensity due to the conjugate heat transfer in AAV.

Thermodynamic and thermophysical effects enabling high-forced convection heat transfer coefficients in supercritical fluids

The strong variation of thermophysical properties of working fluids operating in the vicinity of the critical point makes this thermodynamic domain attractive to several energy applications. Therefore, herein a two-dimensional numerical method is used to investigate the effect of local thermophysical property variations on the local and overall thermal performance of internal convective heat transfer in a pipe in 324 operational conditions. Focusing on carbon dioxide and water as heat transfer fluids, an association of the variation of key thermophysical properties with thermohydraulic performance metrics is proposed, namely: (a) the local and (b) mean convective heat transfer coefficient and (c) the maximal temperature obtained at the tube wall. It is shown that there is an optimal combination of parameters such as mass flow rate, operating pressure, wall heat flux, and inlet temperature that, when properly selected, allow for a minimal maximal wall temperature. As expected, optimality is strongly associated with the Widom—or pseudo critical—line that extends from the critical point. Interestingly, however, contrary to what is observed in constant-property fluids, high heat transfer coefficient or minimal maximum temperature lead to different sets of optimal operating conditions. This difference is explained by how thermophysical properties vary locally along heat exchangers, which significantly affects overall heat transfer.

Numerical study on the thermal-hydraulic performance of a circular-arc-wavy helically coiled heat exchanger developed based on chaotic advection

A circle-arc-wavy helically coiled pipe heat exchanger (CAWC) was proposed and numerically studied. The CAWC is formed by dividing a conventional helically coiled heat exchanger into several (Nbend) segments uniformly and rotate each segment upward and downward alternatively around the chord of the arc with an angle φ. Water under atmosphere pressure was employed as the working fluid. The results showed that the local convective heat transfer coefficient was extremely sensitive to the inlet boundary conditions and showed stochastic characteristics. The mechanism of convective heat transfer enhancement induced by chaotic advection was analyzed using an improved field synergy analytical method. The results showed that the local and total convective heat transfer coefficients always coincided with the variation of the h′ (ρuCpcos(θ);θ: synergy angle) field, suggesting that the enhancement of the h′ field is the main reason for convective heat transfer enhancement induced by chaotic advection. Parameter analyses on the CAWC were conducted with Nbend = 4, 6, and 8; φ = 30°, 60°, 120°, and 180°; Re = 200, 400, 800, 1600 and 3200. It was found that under the same pumping power and heat exchanging area, the total convective heat coefficient of the CAWC can be increased by 40%, relative to the conventional coiled heat exchanger.

Energy Transfer Enhancement Inside an Annulus Using Gradient Porous Ribs and Nanofluids

Abstract Porous media and nanofluid utilization are two passive heat transfer improvement tools, which have been employed extensively in recent years. Porous media with gradient properties result in both a higher effective thermal conductivity and better local convective heat transfer because of conducting the flow to the desired regions. In this study, distinct porous ribs are located on the internal border of an annulus. Four different conditions are considered for permeability change of ribs, including the minimum and maximum Darcy numbers and linearly increasing or decreasing variation in the radial direction, called LIV and LVD, respectively. In the first step, effects of porous rib relative height, porous rib porosity, and flow Reynolds number on the thermal efficiency and pressure drop are investigated. The results show that the configuration with Da = LVD and W/Rh = 0.25 has the maximum performance number PN = 2, that is the Nusselt improvement over pressure drop increment. Porous ribs arrangement with W/Rh = 0.25 and the minimum porosity (ɛ = 0.9) give the best PN. In the next step, the effects of nanoparticle addition with different volume fractions to the base fluid in different Reynolds numbers are investigated. In this step, porous rib relative height is set to W/Rh = 0.25. The results show that the maximum volume fraction has the highest heat transfer enhancement (about 2–2.5 times) but the lower volume fractions have higher PNs (PN ≈ 2.5 at ϕ = 1% and Re = 500).