Variable Heat Transfer Research Articles

Identifying Efficient Cooling Approach of Cylindrical Lithium‐Ion Batteries

With the rapid rise in the application of the lithium‐ion battery (LIB), especially in the automotive sector, its thermal management has become an important concern. Thermal management of LIB requires numerical modeling efforts to acknowledge the battery thermal behavior at different charging and discharging rates to validate the experimental outcome and hence real performance. The present research work discusses a numerical modeling approach to simulate the temperature distribution inside a cylindrical LIB using “finite difference analysis” (FDA) formulations. A multipartition heat generation model is implemented to investigate the thermal behavior of the LIB at different discharging rates. The numerical results are validated by published data for the Li‐ion 18650 cell. Temperature distribution inside the LIB for “both‐tab” and “radial cooling” is compared. The results demonstrate the effectiveness of the “both‐tab” surface cooling approach over “radial cooling” at two different discharge rates (1 and 2 C) with varying heat transfer coefficients (25, 50, and 100 W m−2 K−1). The reduced axial temperature differences of approximately 50%, i.e., 1.9 °C, in the LIB through “both‐tab” cooling compared to radial cooling indicate the significance of the choice of cooling approach.

An Optimisation Algorithm for Detailed Shell-and-Tube Heat Exchanger Designs for Multi-Period Operation

Heat exchangers (HEs) are crucial processing units in industrial plants. Heat exchanger networks (HENs) are often designed for nominal operation. However, processes are becoming increasingly dynamic and should be able to operate over a range of operational periods to address changes in market, seasonality, and start-up and shutdown. HEN synthesis has been widely studied, however most approaches use simplified HE models for optimisation and analysis of the structures, assuming the largest area across all periods of operation results in a feasible HE. The detailed HE design, which includes many more practical constraints, such as variable heat transfer coefficients that are a function of velocity, may result in certain exchangers being infeasible in some operational periods. In this study, an HE design algorithm is proposed, which finds optimal shell-and-tube HEs that are feasible across any number of operational periods, which may involve different duties and fluid properties. This is the first such design algorithm presented in literature. The algorithm works via a smart enumeration algorithm, which solves a nonlinear programming (NLP) optimisation subproblem for each combination of discrete decisions (number of baffles, stream allocation, tube diameters, etc.). Each NLP solves Bell Delaware design equations across all considered periods and allows stream splitting and HE bypassing to find an optimal multi-period HE. If no feasible HE is found, a permutation algorithm is used to find the optimal combination of HEs that can fulfil the required heat duties. The algorithm is demonstrated on two examples, showcasing its performance. Future work is suggested to increase the algorithm’s computational efficiency and to include it in multi-period HEN synthesis.

Experimental analysis on the evaporator startup behaviors in a trapezoidally grooved heat pipe

The startup behaviors of evaporation heat transfer in a trapezoidal groove are experimentally studied by using an axial heat pipe with trapezoidal grooves. The effects of heat load and inclined angles on the evaporation startup behaviors are analyzed and discussed. The startup regime diagram is provided to quantitatively describe the behaviors of evaporation heat transfer based on the Weber number and Bond number. The results indicate that, only the gradual startup is observed for the evaporation heat transfer in trapezoidal grooves, however the gradual startup and overshoot startup is experienced in sequence in the tilt condition as the heat load increases. The appearance of evaporation startup regime transition from the gradual startup to overshoot startup is induced by the evaporation regime transition from the fin-film evaporation to the corner-film evaporation inside the evaporator section. The gradual startup occurs for a small Weber number and the overshoot startup occurs for a large Weber number. Increase in Bond number introduces a larger transition Weber number for the startup regime variation of evaporation heat transfer in a trapezoidal groove. Interestingly, the occurrence of the temperature overshoot of few Celsius degrees can be observed to verify whether the grooved heat pipe is in a horizontal position.

Conjugate Effect on the Thermal Characteristics of Air Impinging Jet

A computational fluid dynamics (CFD) investigation to determine the conjugate heat transfer (CHT) effect on the stagnation and local thermal characteristics due to an impinging process has been carried out in this study using STAR-CCM+ - Siemens PLM commercial code. The transient Navier-Stokes’s equations are numerically solved using a finite volume approach with k-ω SST eddy viscosity as the turbulence model. A fully developed circular air jet with different Reynolds numbers, impinging vertically onto a heated flat disc with different metals, thicknesses, and boundary heat fluxes are employed in the current study to examine the thermal characteristics and provide an enhanced picture for the convection mechanism that used in jet cooling technology. It is found that the thermal characteristics are influenced by the thermal conductivity and thickness of the target upon using air as a cooling jet. The CHT process enhances the local convective heat transfer at the fluid-solid interface due to the variation in transverse and axial conductive heat transfer inside the metal up to a certain redial extent from the stagnation region compared to the process with no CHT. The extent of the radial enhancement depends on the thermal conductivity of the metal. For a given thermal conductivity, the CHT process acts to increase the temperature and convective heat flux of the stagnation region as the metal thickness increases.

Vision sensing and feedback control of weld penetration in helium arc welding process

Consistent joint penetration is essential to mechanical properties of weldments, yet difficult to realize in gas tungsten arc welding due to heat accumulation, varying welding conditions, and uncertain disturbance, especially for manufacturing large-scale structural components. The solution is to monitor welding penetration in real-time and implement closed-loop control. In this paper a directional light-assisted vision sensing scheme is proposed to measure the transient profile of the backside weld pool/bead for monitoring and further controlling the joint penetration state. The developing zone and the developed zone of the backside weld bead are recognized, helping determining its final width. And the relationship between the transient backside bead width and the corresponding final width is established. Furthermore, the backside bead profile is employed as a feedback signal and the closed-loop control of weld formation is implemented to achieve full penetration and a constant backside bead width. A multiple-input single-output model predictive controller is utilized in the control system, creatively combined with an anti-windup integrator eliminating the steady-state error caused by nonlinearity in welding. Experimental results have proved that the developed control system is effective in keeping a consistent backside bead width with a rapid response, a small overshoot and robustness against varying heat transfer conditions.

Flow structure and surface heat transfer from numerical predictions for a double wall effusion plate with impingement jet array cooling

To provide additional understanding of double wall cooling arrangements, especially local distributions of flow properties which are responsible for hot-side surface and cold-side surface heat transfer variations, investigated are numerically-simulated distributions of turbulent flow structural characteristics. Also considered are numerically-simulated surface heat transfer characteristics, including comparisons with experimentally-measured distributions. The numerical results are obtained using the ANSYS FLUENT Version 19.1 numerical code, with a k-ω SST turbulence model. The present arrangement includes a full-coverage effusion cooling plate, with coolant initially supplied by an impingement jet array. Considered are the effects of effusion blowing ratio, impingement jet Reynolds number, and streamwise development on flow structure, and on hot-side and cold-side surface heat transfer characteristics. Data are obtained for an approximately constant main flow Reynolds number of 138000 to 159000, with blowing ratios from 2.2 to 8.5, which correspond to impingement jet Reynolds numbers from 7360 to 25200. Of particular importance are horseshoe-shaped vortices, which form upstream and around each effusion jet, which then result in two horseshoe-originated vortex legs which advect downstream along coolant trajectories. Also important are secondary flows, local flow separation regions, and local velocity regions, which are evident within the effusion hole passages near entrance locations. Within the cross flow passage, locally increased flow velocities, associated with impingement jet locations, are apparent, especially near the lower surface of the cross flow passage. These are associated with regions of augmented streamwise vorticity, which are present around the circumference of each impingement jet.

Nanoliquid Jet Impingement Heat Transfer for a Phase Change Material Embedded Radial Heating System

Abstract Nanoliquid impingement heat transfer with a phase change material (PCM) installed radial system is considered. The study is performed by using the finite element method for various values of Reynolds numbers (100 ≤ Re ≤ 300), height of PCM (0.25H ≤ hpcm ≤ 0.75H), and plate spacing (0.15H ≤ hs ≤ 0.40H). Different configurations using water, nanoliquid, and nanoliquid + PCM are compared in terms of heat transfer improvement. Thermal performance is improved by using PCM, while best performance is achieved with nanoliquid and PCM-installed configuration. At Re = 100 and Re = 300, heat transfer improvements of 26% and 25.5% are achieved with the nanoliquid + PCM system as compared to water without PCM. The height of the PCM layer also influences the heat transfer dynamic behavior, while there is 12.6% variation in the spatial average heat transfer of the target surface with the lowest and highest PCM heights while discharging time increases by about 76.5%. As the spacing between the plates decreases, average heat transfer rises and there is 38% variation.

Experimental and Numerical Analysis of a Plate Heat Exchanger Using Variable Heat Transfer Coefficient

Thermal design and analysis of heat exchangers are predominantly conducted considering constant heat transfer coefficients. However, these vary along the length and affect the calculations of heat transfer rates and area allocations. The current paper investigates the variations in the heat transfer coefficients in plate heat exchangers (PHX), using different numerical approaches. The heat transfer coefficient is calculated at the inlet, outlet, and systematically selected intermediate points for each method. The analysis is conducted for two different systems, i.e., a laboratory-scale and an industrial scale PHX at different chevron angles. It is concluded that the effect of the variable heat transfer coefficient is more significant for the large-scale heat exchanger due to high flow rates, geometrical specifications, Reynolds number, and thermophysical properties. The deviation of the local heat transfer coefficient along the heat exchanger length is approximately 9–14% and 3–6% for industrial and laboratory scale PHX, while an area deviation of around 15% is observed.

Adaptive opaque façades and their potential to reduce thermal energy use in residential buildings: A simulation-based evaluation

Adaptive façades are a promising choice to achieve comfortable low-energy buildings. Their effective performance is highly dependent on the local boundary conditions of each application and on the way the dynamic properties are controlled. The evaluation of whole building performance through building performance simulation can be useful to understand the potential of different Adaptive opaque façades (AOF) in a specific context. This paper evaluates through dynamic simulations promising design solutions of AOF for a residential building use in six different climates. It quantifies the total delivered thermal energy of 15 typologies of AOFs which consist of alternative adaptation strategies: (i) variation of solar absorptance of the cladding, (ii) variation of the convective heat transfer of air cavities and (iii) adaptive insulation strategies. For the first time, it also quantifies the performance of AOF which combine more than one adaptation strategy. The results show that the variation of the heat transfer by means of Adaptive Insulation components has the most significant impact on the reduction of the thermal energy use. The variation of the solar absorptance has also a significant positive impact when reducing heating consumption, but only if this adaptation strategy is actively controlled and combined with Adaptive Insulation components.

Development of on-line neural network based adaptive fractional-order sliding mode robust controller on electromechanically actuated engine cooling system

The reduction of temperature fluctuations around desired reference and control input energy for cooling actuators are among the most important aims of engine thermal management system. However, maintaining of engine/radiator outlet temperature within certain limits is a challenging process in different engine operating conditions due to unknown in-cylinder variable heat transfer rate in terms of the control theory. For this purpose, the thermodynamic model is obtained by dividing into two subsystems, such as the engine and the radiator subsystem in the presence of unknown heat transfer rate, external disturbance, and uncertainties in this study. To increase system robustness, tracking engine inlet/outlet temperature control of engine cooling components is performed using the adaptive fractional-order sliding mode control structure. Moreover, the disturbances acting on the system and unknown heat transfer dynamics of the system are estimated with a radial basis function on-line neural network estimator. The efficiency of proposed controller strategy for electromechanically actuated engine cooling system is compared by proportional–integral–derivative and classical integral sliding mode control under the NEDC (New European Driving Cycle) and WLTP (Worldwide Harmonized Light Vehicles Procedure) transient operating conditions. The temperature error tracking performance is tested by paying attention integral square error, integral absolute error, integral time-weighted absolute error along to the driving cycles. The obtained results showed that the adaptive fractional-order sliding mode control–based engine cooling system outperforms compared to the optimally gain tuned proportional–integral–derivative and integral sliding mode control schemes in terms of reducing of tracking error and engine cooling actuators energy consumption for all engine operating cycles.

Numerical optimization of the fin shape experiments of a heat conjugate problem surface air/oil heat exchanger (SACOC)

Efficient aircraft engine designs imply the presence of a surface air/oil heat exchanger placed at the bypass flow separation of the engine. The heat exchanger consists of several parallel longitudinal fins that increase the contact area to obtain a higher heat dissipation rate. The design of these fins is an important task as the pressure drop generated by the presence of the heat exchanger is the largest drawback to be minimized in the final setup. Consequently, an efficient design of the fins that optimizes the global performance of the heat exchanger is highly demanded. The result of this optimization process should minimize the pressure drop caused by the exchanger without decreasing the heat transfer. The optimization methodology proposed in this work is split in two independent parts: in the first one, the fin shape is optimized using the adjoint method and the derived sensitivity function that controls the position of the fin shape design. In parallel, the range of fin thickness and distance between fins was investigated to have an optimized fin distribution for a given SACOC width. Once the geometry was optimized, the coupled conjugate heat problem is numerically solved using realistic conditions showing good accuracy in the two presented validations. The result for the optimized geometry presents a substantial pressure drop reduction with little heat transfer variation, addressing the objectives that motivated the present study.

Thermal management of nanoliquid forced convective flow over heated blocks in channel by using double elliptic porous objects

Effects of using nanoliquid and double porous elliptic objects on the flow separation and convective heat transfer control for flow over multiple heated blocks in channel are numerically investigated by using finite element method. Spherical, brick, blade and cylindrical shaped alumina particles are used in water up to solid volume fraction of 2%. Impacts of Reynolds number (50≤Re ≤ 500), permeability of the porous ellipses (10−5≤Da1≤5 × 10−2 and 10−5≤Da2≤5 × 10−2), aspect ratio of the ellipses (0.25≤AR1≤1.5, 0.25≤AR2≤1.5), distance between the ellipses (0.2H ≤ sx ≤ 1.6H) and distance of the first ellipse from inlet (−H ≤ mx ≤ H) on the hydro-thermal performance are explored. The highest performance improvement is obtained by using the cylindrical shaped particles in the base heat transfer fluid at the highest solid volume fraction and the amount of heat transfer enhancement are 33% and 40.5% for hot blocks while negligible pressure drop is observed. Varying the aspect ratio of the ellipses in the perpendicular direction to the flow, up to 25% and 30% heat transfer increment are obtained for hot block with slight rise of pressure coefficient. However, when varying the horizontal location of the porous ellipses in the channel and permeability of them, there is considerable rise of pressure drop. Variation of the average heat transfer from the hot blocks with varying permeability of the ellipses are obtained as 32% and 25%. Successful hydro-thermal performance estimation results are achieved by using artificial neural networks with feed-forward network architecture of 1 hidden layer and 17 neurons.

Acoustic and Thermal Correlation for a Building’s Envelope in a Mediterranean Climate in Morocco

The aim of this paper is to present a comparative analysis of the thermal and acoustic insulations of four different buildings' envelopes: Single layer clay brick wall (Wall(a)), Single layer concrete brick wall (Wall(b)), double layer hollow brick wall with a medium of polystyrene (Wall(c)) and double layer hollow brick wall with a medium of air (Wall(d)), which are all modelled and simulated by using finite element method (FEM) under different climatic conditions. Moreover, the study of noise transmission through these walls was released by placing a linear nonmonochromatic source of sound 1m away from the exterior side of each wall and it is emitted sound waves with a velocity of 200 m/s with low frequencies, while an acoustical comparative analysis was investigated under the same thermal conditions as the two previous steady states study. The analysis of the heat transfer and sound pressure level (SPL) variations through the walls (a), (b), (c) and (d) shows that the wall (d) is a good thermal and sound insulator compared to the walls (a), (b) and (c) due to the insulation by air gap with a thickness of 0.06 m, it provides a high rate of sound pressure level attenuation compared to the other walls under study. Finally, the insulation by air-gap gives is good enough to ensure simultaneously energy savings and acoustic comfort.

Analysis of Non-Symmetrical Heat Transfers during the Casting of Steel Billets and Slabs

The current automation of steelmaking processes is capable of complete control through programmed hardware. However, many metallurgical and operating factors, such as heat transfer control, require further studies under industrial conditions. In this context, computer simulation has become a powerful tool for reproducing the effects of industrial constraints on heat transfer. This work reports a computational model to simulate heat removal from billets’ strands in the continuous casting process. This model deals with the non-symmetric cooling conditions of a billet caster. These cooling conditions frequently occur due to plugged nozzles in the secondary cooling system (SCS). The model developed simulates the steel thermal behavior for casters with a non-symmetric distribution of the sprays in the SCS using different boundary conditions to show possible heat transfer variations. Finally, the results are compared with actual temperatures from different casters to demonstrate the predictive capacity of this algorithm’s approach.

On-Line Control of Stresses in the Power Unit Pressure Elements Taking Account of Variable Heat Transfer Conditions

Coal-fired power units, now balancing power shortages in the power system, must be characterised by increasingly higher flexibility of operation. This means faster start-ups and the capacity for frequent decreases and increases in the power output. These processes cause large temperature gradients in elements of the power unit and the turbine and lead to an increase in the stress level. At such an operating regime it is impossible to ensure safety based on start-up characteristics only—it becomes necessary to constantly monitor stress levels in critical areas of machinery and equipment elements. The stress level in turbine elements can be monitored on-line using algorithms based on Green’s functions and Duhamel’s integral. This paper presents examples of modifications of stress calculations in turbine valves and casings during start-ups. By modifying basic algorithms, it is possible to take into account the impact of the variability of heat transfer coefficients on the thermal stress level. Additionally, individual Green’s functions and correction factors were determined for specific stages of start-ups. Due to modifications, it is possible to obtain satisfactory agreement with the results obtained from FEM-based calculations for the entire heating process. Equations are also given that enable estimation of values of the heat transfer coefficient in turbine valves. The proposed modification of the algorithm will substantially improve the accuracy of stress modelling in transient states of the turbine operation. On-line stress monitoring will enable an increase in the flexibility of the power unit operation and facilitate operational control, ensuring safety of individual elements at the same time. The stress values calculated in the on-line mode can also be used to estimate fatigue life consumption and forecast the residual lifetime of individual components.

Analysis of convective condensation heat transfer for moist air on a three-dimensional finned tube

The condensation of moist air plays an important role in daily life and industry. During the condensation process, single-phase and phase-change heat transfers occur simultaneously. A three-dimensional finned tube is an effective element for enhancing heat transfer. However, the latent and sensible heat transfer during moist air condensation outside a three-dimensional finned tube has not been experimentally studied. In this study, experiments were performed using a three-dimensional finned tube to enhance the moist air condensation process. For a better understanding of the enhancement mechanism, latent and sensible heat transfer variations with relative humidity ranging from 0.75 to 0.95 and Reynolds numbers ranging from 838 to 3444 were obtained and analysed. A smooth tube was used for the comparison. The experimental results indicated that the three-dimensional finned tube was favourable for the enhancement of moist air condensation, especially for latent heat transfer. When the Reynolds number was 2504, the latent and sensible heat transfers of the three-dimensional finned tube were 43.6%–81.8% and 15.6%–22.9% higher than those of the smooth tube, respectively. In addition, the latent and sensible heat transfers of the three-dimensional finned tube and the smooth tube were obtained under different velocities of moist air, and empirical correlations were developed to predict the sensible and latent heat transfers with deviations of less than ± 20%. The results obtained can provide a reference for the design of three-dimensional finned tubes used in moist air condensation.

Nonlinear heat transfer analysis of spines using MLPG method

In the current article, an analysis of heat transfer in the spine of different profiles is carried out. A nonlinear differential equation that includes the effect of temperature-dependent thermal conductivity, variable heat transfer coefficient, and surface radiation is solved using a predictor-corrector scheme and the meshless local Petrov-Galerkin (MLPG) method. The moving least square (MLS) method is employed for the approximation of unknown function and the penalty parameter method is used to impose essential boundary conditions. Temperature variation along the length of the spine and rate of heat transfer in steady-state, under convective and convective-radiative environment has been obtained and explained. Performance of the spine under the above conditions is evaluated. Further, the study is extended to calculate volume adjusted performance which presents a comparative thermal performance of different profiles.

The Role of Secondary Flows and Separation in Convective Heat Transfer in a Rotating Radial Vane Brake Disk

Abstract This study presents, for the first time, distributions of local internal temperature and convective heat transfer in a rotating radial vane brake disk and explains mechanisms in conjunction with secondary flows and flow separation within its ventilated coolant passages. In particular, variations of radial, circumferential (vane-to-vane), and axial (inboard-to-outboard) heat transfer on internal end-wall surfaces, and their alteration due to varying number of radial vanes and rotating speed are experimentally detailed. It has been demonstrated that conventional ventilated radial brake disks where the air inflow is drawn from the inboard face are likely to suffer substantial axial variations of temperature and heat transfer between the inboard and outboard disks, which possibly exacerbates thermal distortion (i.e., coning). Further, for a typical number of vanes (i.e., 36 vanes) used on automobiles, internal thermal distributions are highly nonuniform. However, the thermal end-wall uniformity improves considerably as the number of vanes is increased to say 72 vanes. Specifically, as the number of vanes is increased, secondary flow mixing enhances overall convective heat transfer and improves thermal uniformity. In contrast, separation causes large end-wall thermal nonuniformities in radial and circumferential distributions between the pressure side and the suction side of radial vanes. This effect nonetheless also decreases as the number of vanes is increased.

A quantitative phase diagram of droplet impingement boiling

The traditional phase diagram of droplet impingement boiling on a heated substrate is drawn based on the boiling morphologies recorded with high-speed camera, which is useful to qualitatively determine the strength of droplet impingement heat transfer and the transition boundaries. However, the major shortcut of this method is that one cannot distinguish the heat transfer variation in the same region on phase diagram. To address this problem, this paper proposes a quantitative phase diagram which is determined with the measurements of temperature evolution in substrate. In the experiments, the two ends of a differential thermocouple are inserted in the substrate at different depth, and the recorded maximum temperature difference during the droplet impact is employed to quantify the intensity of heat transfer. Therefore, the contour of the maximum temperature difference in the space of the substrate temperature and the droplet Weber number provides a quantitative phase diagram. The comparisons of the two phase diagrams are conducted, and it shows that the new phase diagram could reveal richer information. For example, a second peak of the heat flux in the transition boiling could be observed under specific Weber numbers, which forms an island in the quantitative phase diagram. Finally, based on the captured images, the observed heat transfer characteristics were analyzed and explained.

Influence of system buoyancy and hydraulic diameter on turbulent mixed convection in a rectangular duct

We investigated buoyancy-aided, turbulent mixed-convection heat transfer in a vertical rectangular duct. In order to achieve highly buoyant conditions, mass transfer experiments instead of heat transfer ones were performed based on the analogy between heat and mass transfer. Reynolds numbers ranged from 2,700 to 25,450 and Grashof numbers were 3.30 × 105 and 5.15 × 106. The results of turbulent forced-convection experiments were consistent with previous studies. Notably, at the same axial position, local heat transfer was affected by downstream flow for different heated lengths, and heat transfer was affected by the hydraulic diameter. Five distinct flow regimes were observed based on buoyancy coefficients. Particle image velocimetry measurements revealed that variations in local heat transfer were closely related to the amount of turbulence produced, which was affected by differences in velocity between buoyant and forced flows. Turbulent mixed-convection heat-transfer correlations are proposed based on analytical models and parametric studies of the influence of system buoyancy and hydraulic diameter.