Heat Flux Transfer Research Articles

Numerical and geometric investigation of pool boiling heat transfer in cavities with isothermal rectangular corrugations

This numerical work presents a geometrical investigation of a corrugated isothermal surface placed in a two-dimensional cavity subjected to unsteady, turbulent pool boiling flows. The main purposes are maximizing the heat transfer rate between the isothermal surface and the surrounding water flow, and the volume of vapor generated into the cavity. The geometric investigation followed the constructal design method, being the ratio Hi/Li (i = 1, 2 or 3) of the corrugations varied for three different numbers of corrugations: N = 1, 2, and 3, keeping constant the corrugations area. The volume of fluid (VOF) and Lee's evaporation-condensation models are used to estimate the volume fractions of water vapor/liquid and interfacial mass transfer. The unsteady Reynolds Averaged Navier Stokes (URANS) continuity, momentum and conservation of energy equations, and volume fraction transport equation, are solved using the finite volume method (FVM) available in software Ansys FLUENT. For closure of turbulence, the k - ɛ model is adopted. For validation of the model, the heat flux and convection heat transfer coefficient obtained for a pool boiling bared surface case are compared with Rohsenow's correlations, and differences lower than 7.0 % are reached. Results indicated a strong influence of the ratio Hi/Li and number of corrugations (N) in the heat transfer rate per unit depth (qs) and dimensionless volume of vapor (Vdim) generated into the cavity. The highest intrusion of the corrugations led to the generation of few large and many small scales, benefiting the thermal performance, regardless of the performance indicator employed. The optimal configuration, N = 3 and H3/L3 = 2.0 improved 49 % and 188 % the Vdim and qs compared with the worst corrugated case, showing the importance of the geometry of the corrugation in this problem.

Investigation of heat transfer characteristics and optimization design of platen superheaters in the furnace of a 350 MWe supercritical CFB boiler

With the scale-up of circulating fluidized bed (CFB) boilers, the size of the platen heating surface within the furnace increases. Deformation, crack, and tube explosion of the platen heating surface threaten the safe and stable operation of CFB boiler. Large wall temperature deviation and overtemperature of partial tubes of heating surfaces are two of main reasons resulting in above problems. To identify the causes of large wall temperature deviation and decrease them, the heat transfer characteristics and wall temperature distribution uniformity of platen superheaters in the furnace of a 350 MWe supercritical CFB boiler were studied by experimental tests, and the structure of the high temperature superheater was optimized by numerical simulation to improve the temperature distribution uniformity. The steam parameters and wall temperatures of the platen superheaters were measured at boiler loads of 50 % BMCR, 75 % BMCR, and 100 % BMCR. The heat transfer coefficient, heat flux, and wall temperature uniformity of the platen superheaters were analyzed. At 100 % BMCR boiler load, the average heat transfer coefficient and heat flux of the medium temperature superheater IIs were 165.5 W/(m2·°C) and 56 kW/m2, respectively; while those of the high temperature superheaters were 179.7 W/(m2·°C) and 53.2 kW/m2, respectively. The wall temperature distribution uniformity among six medium temperature superheater IIs or six high temperature superheaters was relative good, while the temperature distribution uniformity of a single platen superheater required improvement. An optimized structure of the high temperature superheater was obtained from eight structures by numerical simulation, achieving a steam temperature deviation of less than 8 °C at the boiler load of 100 % BMCR.

Optimization of wavy geometries for enhanced condensation heat transfer on vertical plates: An experimental study

In this study, macro-scale wavy geometries are used to enhance condensation heat transfer on vertical plates. Three wavy plates with different amplitudes (3.33 mm, 4.17 mm, and 5.56 mm) were compared to a flat plate over a temperature difference range of 12K–50K. For each plate configuration, heat transfer coefficient (HTC), enhancement factor (EF), heat flux (q), and Reynolds number (Re) were measured. Several studies have demonstrated that wavy surfaces offer better performance than flat surfaces, with the highest performance showing up for the 4.17 mm amplitude plate (Wave-2). At higher temperature differences, this optimal configuration achieved enhancement factors up to 1.31. As a result of the study, Reynolds numbers for the largest amplitude plate were 2.4 times higher than for the flat plate, indicating that increasing wave amplitude does not necessarily result in better heat transfer. These findings provide valuable insights for optimizing heat exchanger design in various industrial applications, offering a practical and cost-effective approach to improving falling film condensation heat transfer efficiency.

Enhanced Phase Change Heat Transfer with Fused Deposition Modeling (FDM) Printed Pit and Pillar (Pi2) Arrays

Phase change heat transfer, crucial in thermal management systems, can be significantly enhanced through optimized surface structures. This study investigates pool boiling heat transfer enhancement using 3D printed structures with carefully designed pillar and pit geometries. We present a novel approach combining the Dual Rise model with separate liquid–vapor pathways to improve Critical Heat Flux (CHF) and Heat Transfer Coefficients (HTC). Using copper-infused Polylactic Acid (PLA) filaments, we created and sintered structured surfaces featuring pit-assisted nucleation sites, interpillar spacing for vapor escape, and pillar roughness for enhanced liquid supply. Experiments with deionized water and ethanol under atmospheric pressure demonstrated substantial improvements over plain surfaces: water showed an 87% increase in CHF and 39% in maximum HTC, while ethanol exhibited even greater enhancements of 122% in CHF and 61% in HTC. These improvements are attributed to the synergistic effects of optimized surface geometry and separated liquid–vapor pathways, reducing counterflow resistance and improving hydrodynamic stability. A theoretical framework based on the Dual Rise model explains these enhancements, providing insights into coupled capillary action and hemiwicking effects in boiling heat transfer. The study introduces predictive models for CHF and HTC enhancement, offering valuable tools for future design optimization in applications ranging from electronics cooling to power plant thermal management and advanced heat exchangers.

Surface structures and wettability: Their roles in enhancing nucleate boiling heat transfer of refrigerants

Nucleate boiling is an effective heat transfer mechanism. Previous studies have demonstrated that modifying surface structure or wettability can independently improve nucleate boiling efficiency; however, the combined effects and optimal modification sequence remain underexplored. This research aims to comprehensively analyze the influence of surface structure and wettability on the nucleate boiling heat transfer performance of refrigerants. Different surface structures and wettability were prepared using surface polishing, laser ablation, mechanical cutting, and a combination that involved initially employing laser ablation, followed by the application of chemical etching, and finishing with the treatment using materials possessing low surface energy. The static contact angles on these surfaces were measured via the capillary rise method, followed by pool boiling experiments and bubble behavior visualization. The results show that increasing surface wettability can enhance bubble departure frequency and capillary wicking, thereby improving critical heat flux and heat transfer coefficient. The key to optimizing surface modification to enhance critical heat flux involves constructing alternating high and low nucleation core regions to reduce interactions between bubbles; for enhancing heat transfer coefficient, the key lies in constructing compact nanoscale structures that expand the heat transfer and nucleation area, followed by improving wettability.

Study on cooling efficiency and mechanism of lithium-ion battery thermal runaway inhibition by additives enhanced water mist based on boiling heat transfer theory

In recent years, fire and explosion accidents caused by thermal runaway (TR) of lithium-ion batteries (LIB) have occurred frequently, which has become one of the main obstacles restricting its development. Therefore, it is urgent to develop fire extinguishing agents with high cooling efficiency to inhibit the growth of TR. The water mist is an effective choice, but it shows poor performance with the rise in cell temperature under TR. In this study, two additives, KCl and the nonionic fluorocarbon surfactant FC-4430, were added to the water mist to improve the cooling efficiency. In addition, the inhibition effect exerted by the modified water mist during the whole process of lithium-ion battery TR was experimentally investigated. Furthermore, the influence of water mist with additives on the Leidenfrost effect in the high temperature range was deduced, and the reinforcement cooling mechanism was analyzed by heat transfer theory. The results show that KCl and FC-4430 can increase the initial cooling rate in the high temperature range by 48.6%–120%. The reinforcement mechanism is attributed to changing the medium's physical parameters with additives. On the one hand, the Leidenfrost point of the droplets on the cell surface is increased, which in turn extends the temperature range of the nucleate boiling stage, and on the other hand, the heat flux density and heat transfer of the water mist in the film boiling state are increased.

Thermal conductivity impact on MHD convective heat transfer over moving wedge with surface heat flux and high magnetic Prandtl number

The present evaluation focused on heat transfer and magnetic flux along the moving non-conducting wedge shape with thermal conductivity and surface heat flux effects. A suitable and well-known similarity transformation for integration is used for the coupled non-linear partial differential equations with stream formulations. The Keller Box technique is utilized to integrate the final non-similar equations numerically. The discretized algebraic equations are displayed graphically and numerically using the MATLAB software. The physical characteristics like temperature, magnetic field, velocity profiles, skin friction, heat transfer and magnetic intensity are investigated with various factors. The influence of relevant characteristics like thermal conductivity ξ, Prandtl number Pr, wedge parameter β, variable viscosity ε, and Biot number Bi is interpreted graphically and numerically. It is noted that velocity graph effects are declined at lower value of Biot number and enhanced value is obtained at the higher value of Biot number. The fluid temperature with highest thermal slip effects is displayed with minimum value of thermal conductivity but minimum value is obtained at large value of thermal conductivity. The maximum heat flux value is obtained at large value of moving parameter.

Comparison between pool boiling system of graphene quantum dots and nitrogen-doped graphene quantum dots suspended in binary base fluids

Using nanofluids instead of conventional heat transfer fluids as a passive method is a well-established and widely used technique by researchers to increase the rate and thermal performance of engineering equipment. In this study, the hydrothermal method with a bottom-up approach was used for the synthesis of graphene quantum dots and nitrogen-doped graphene quantum dots. Then, nanofluid samples were prepared in a two-step process, by adding nanoparticles to binary base fluids of deionized water and ethylene glycol in volume concentrations of (50:50) and (60:40), in four concentrations of 100, 200, 500, and 1000 ppm. In order to better understand the boiling heat transfer mechanism and measure its characteristics such as critical heat flux and heat transfer coefficient, an experimental system was designed and built. Nanofluids based on graphene quantum dots have unique features such as compatibility with the environment, economic efficiency, high stability and suitable heat transfer capability. For this reason, their selection in the pool boiling heat transfer process, in addition to saving energy, is introduced as one of the most effective options for improving CHF and HTC. The tests were performed under saturated conditions, atmospheric pressure, and on a vertical flat and polished copper thermal plate. Prepared nanofluids GQDs and N: GQDs based on DI-water maintained their apparent stability for two months. For GQDs nanofluids at an optimal concentration of 500 ppm with a volume ratio of (60:40) DI-water and EG, compared to DI-water, the greatest increase in CHF and HTC is 90.69, 85.011 % and for N: GQDs at a concentration of 500 with a volume ratio (50:50) DI-water and EG, 75.37 and 78.17 % compared to DI-water.

Efficient natural gas combustion and NOx reduction: A novel application of simultaneous over-fire air (OFA) and flue gas recirculation (FGR) in a 250 MW opposed firing steam power plant boiler

OFAGR, as a new controllable technology, combines over-fire air (OFA) and flue gas recirculation (FGR) systems for modifying combustion characteristics and enhancing boiler efficiency. It also reduces NOx generation considerably. The effects of OFA and FGR on the combustion are reported for coal-fired boilers, however, no reference was found for the application of OFAGR, especially in boilers with natural gas fuel so far. Furthermore, the method of adjusting the ratio of OFA and FGR in OFAGR nozzles to achieve the required combustion modification is not investigated and this fact should be even covered for different types of boilers. For analysis of complicated combustive mixed fuel (natural gas) and air in furnace, computational fluid dynamics (CFD) simulated the flow by the use of Eddy-Dissipation Concept (EDC) and developing reactions by Jones-Lindstedt mechanism. Simulation results are validated by the measured boiler operation data for a 250 MW steam power plant boiler. The staged combustion in each burner is modeled by considering three inlet air flows (primary, secondary, and tertiary) with swirl. The effects of different ratios of OFA and FGR streams on characteristics of combustion and NOx production are investigated. The combustion efficiency, temperature profile, distribution of species, and the transferred heat flux to water-walls are computed, and analyzed. Results showed injecting only OFA from OFAGR nozzles, reduced NOx generation up to 55.4 % but lowered the furnace efficiency from 95.3 % to 93.1 %. When the OFAGR stream contained 25 % FGR and 75 % OFA (case study 2), an ideal state existed for increasing the combustion efficiency from 95.3 % to 96.0 % and NOx reduction by 58 %. Increasing the share of FGR in the OFAGR stream by more than 25 % improved the NOx reduction but reduced the combustion efficiency from 95.3 % due to the drop in the usable absorption heat of the boiler. Calculations showed that the OFAGR injection stream containing 64.1 % OFA and 35.9 % FGR reduced NOx formation from 274 to 112 ppm without losing furnace efficiency.

On the intrusiveness of phosphor thermometry for metal surface temperature measurements in reciprocating engines: an experimental and heat conduction modeling study

Abstract Lifetime-based phosphor thermometry has been applied in a wide variety of surface temperature measurement applications due to its relative ease of implementation and robustness to background interference when compared to other optical temperature measurement methods. It is often assumed that the technique is minimally intrusive if the thickness of the applied phosphor coating is < 20 µm. To evaluate this assumption, high-speed phosphor surface temperature and thermocouple measurements were performed on 4140 steel substrates installed in the cylinder head of an optically-accessible internal-combustion engine for four operating conditions. For phosphor thermometry measurements, four substrates were studied, each coated with a phosphor layer of different thickness ranging between 6 µm and 47 µm. The phosphor thermometry measured temperature swings during combustion were shown to be heavily impacted by the presence of the phosphor coatings, increasing by roughly a factor of 2 - 2.5 when increasing the thickness from 6 µm to 30 µm. A technique was implemented which utilizes the temperature data in combination with a heat conduction model to provide estimates of the temperature swing and heat transfer flux in the absence of the phosphor coating. It was shown that even the 6 µm phosphor coating could lead to an order of magnitude increase in the temperature swing relative to the uncoated 4140 steel substrate. Despite the intrusiveness of phosphor thermometry for the surface temperature measurements, reasonable agreement was demonstrated between heat flux estimates determined with the heat transfer modeling technique and those deduced from temperature-swing measurements using two different high-speed thermocouples. The results indicate that phosphor surface thermometry can be a reliable surface temperature and heat flux diagnostic for transient high heat flux environments, as long as proper care is taken to account for the impact of the phosphor layer on the measurement.

Quantitative analysis of heat and mass transfer in MoS2-Al2O3/EG hybrid flow between parallel surfaces with suction/injection by numerical modeling of HPM method

This description focuses on how the magnetic field affects mass and heat transfer in a hybrid nanofluid (Hnf) between two parallel, rotating plates. By dispersing aluminum oxide (Al2O3) and molybdenum disulfide (MoS2) nanoparticles (NPs) in ethylene glycol (EG), a hybrid nanofluid (Hnf) is created. This research aims to analyze the heat and mass transfer characteristics in the flow of a hybrid nanofluid (MoS2-Al2O3/EG) between two rotating parallel plates under the influence of a magnetic field. Furthermore, the statistical technique of response surface methodology (RSM) has been employed to optimize the parameters of velocity, temperature, and concentration of the nanofluid within the flow region bounded by the rotating plates. Dimensionless differential equations have been calculated and checked using the Homotopy perturbation method. This study introduces a novel approach by utilizing the RSM method to identify optimal points for velocity and temperature parameters of nanofluids between two stretching plates for the first time. Additionally, the article innovatively applies the exact HPM method to validate dimensionless linear and non-linear coupling equations. As the Reynolds number and the suction/injection coefficient of nanofluids flowing between two plates under tension increase, the results indicate a decrease in the velocity field. This decrease in velocity field can be attributed to the reduction in fluid diffusion as viscous forces diminish with varying Reynolds numbers. The ideal temperature distribution for nanofluids flowing between two parallel plates occurs when they are uniformly dispersed at the midpoint between them. As the distance from the initial point of nanofluid entry to the end of the plates increases, along with the vertical distance from the bottom plate, the temperature gradient diminishes, reducing the thickness of the thermal boundary layer. The velocity gradient and the rate of heat flux transfer between the nanofluid and plate rise by 34 % when the volume percentage is raised from 1 % to 5 %.

Analytical solution to the two-dimensional inverse heat conduction problem in an axisymmetric cylindrical coordinate system

The inverse heat conduction problem involves estimating an unknown cooling or heating action based on internal temperature histories in a body. Such a problem is ill-posed because the estimated results are highly sensitive to input data noise. An analytical solution is developed for the two-dimensional inverse heat conduction problem in an axisymmetric cylindrical coordinate system using the Laplace transform technique. On the basis of the known temperatures at multiple measuring points distributed on two cylindrical measuring surfaces along the axial direction inside the solid, the expressions for the surface temperature and heat flux are explicitly obtained in the form of a power series of time and eigenfunctions of the space variable. A comprehensive estimation error assessment is conducted using the internal temperature distribution that is obtained using a direct heat conduction analysis under the prescribed stationary and moving boundary conditions. The constant thermal condition is well reproduced using the present inverse solution, except in the vicinity of the discontinuity point. In addition, the effective heat flux and effective heat transfer coefficient are introduced to obtain the best estimation of the transient heat transfer in the major cooling area of the rotating cylinder.

Experimental investigation of humidified air condensation in different rows of serpantine heat exchanger – Cooling water flow rate effect

Condensing economizers are used in the industry as they enable the acquisition of additional heat during vapor condensation. In condensing heat exchangers, water is the usual cooling agent due to its high specific heat capacity and thus efficient heat removal. Therefore, experiments of hot humid air condensation in a serpentine economizer being cooled by water were performed to reveal the effect the cooling water flow rate has on humidified air condensation in separate rows of the serpentine type economizer (with vertical tubes). The results have shown that the cooling ratio (mass flow ratio between the coolant and the humid air) had a minor effect on the distribution of the humidified air temperature along test section for inlet Reynolds number of 3000–10000. The effects on the condensation flux distribution in different rows were much stronger. The most optimal cooling ratio was determined to be 3. The results have shown that the biggest condensation efficiency is up to 35%. It was revealed that in some cases the convection was prevailing in the first rows of the economizer and this resulted in a decreased efficiency/performance of the exchanger.The obtained results from the practical point of view will provide an extended basis for the optimisation of the design of economizers for waste heat recovery in the cases of different cooling water flow rates. It could also be applied to validate computational models developed for condensation heat transfer and condensate flux numerical modelling along economizer.

The effects of bioconvection, non‐Fourier heat flux, and thermal radiations on Williamson nanofluids and Maxwell nanofluids transportation with prescribed thermal conditions

AbstractThe utilization of nanoentities in common fluids has opened new opportunities in the area of heat transportation. The rising requirements to enhance the efficiency of compact heat exchangers can be achieved by using various nanofluids. In this article, the thermal output of Maxwell and Williamson nanofluids transport over a prolonging sheet with bioconvection of self‐motivated organisms is scrutinized. A magnetic flux and the porous effects of a medium influence the flow of fluids. The fundamental principles for conservation of mass, concentration, momentum, and energy yield a nonlinear set of partial differential equations that can then be altered into ordinary differential form. A heat transfer flux is presented along with temperature boundary conditions, PST, and PHF (prescribed surface temperature and prescribed heat flux). The numerical results are acquired by executing the Runge–Kutta method with a shooting procedure in MATLAB coding. By fluctuating the inputs of influential variables of the dependent functions, a precise overview of the scheme is acquired. It can be seen that velocity decreases with the rising values of buoyancy ratio, magnetic force, Raleigh number, and porosity. Also, the temperature of the fluids begins to rise directly with the rising values of thermophoresis and Brownian motion parameters. The present study addresses bioconvection, non‐Fourier heat flow, and thermal radiations while combining the special properties of Williamson and Maxwell nanofluids. The field of biomedical engineering may benefit from this study, particularly with regard to therapies for hyperthermia and drug delivery systems. This study can be useful in cutting‐edge cooling systems, bioengineering, solar energy conversion and biotechnology.

Experimental investigation on spray cooling heat transfer properties of ethylene glycol-water-based nanofluids

Abstract Spray cooling is a practical solution for high heat flux heat dissipation problems. The spray cooling experiment was conducted with 50wt% ethylene glycol aqueous solution as the base liquid in this paper. The effects of the concentrations of copper nanoparticles, copper oxide nanoparticles, silica nanoparticles and surfactant Tween-20 on the heat transfer characteristics during spray cooling were investigated. The results showed that adding nanoparticles within a certain concentration range could enhance the effectiveness of spray cooling's heat transmission. Under the same conditions, adding copper nanoparticle has the best effect, followed by copper oxide nanoparticle, and silica nanoparticle has relatively poor effect. Among them, the heat transfer coefficient and heat flux of the nanofluid could be raised by 10.58% and 11.34% in comparison to the base liquid when the concentration of copper nanoparticles was 0.01wt%. As the concentration of nanoparticles rises higher, spray cooling's ability to transfer heat is hampered. Adding the surfactant Tween-20 could effectively improve the copper-water-ethylene glycol nanofluid's capacity for heat transmission. When the concentration is 3ppm, the heat transfer coefficient and heat flux of the nanofluid with Tween-20 were increased by 9.81% and 10.38% compared to the copper-water-ethylene glycol nanofluid.

Spatiotemporal Surface Temperature Measurements Resolving Flame-Wall Interactions of Lean H2-Air and CH4-Air Flames Using Phosphor Thermometry

AbstractSpatiotemporal wall temperature (Twall) distributions resulting from flame-wall interactions of lean H2-air and CH4-air flames are measured using phosphor thermometry. Such measurements are important to understand transient heat transfer and wall heat flux associated with various flame features. This is particularly true for hydrogen, which can exhibit a range of unique flame features associated with combustion instabilities. Experiments are performed within a two-wall passage, in an optically accessible chamber. The phosphor ScVO4:Bi3+ is used to measure Twall in a 22 × 22 mm2 region with 180 µm/pixel resolution and repetition rate of 1 kHz. Chemiluminescence imaging is combined with phosphor thermometry to correlate the spatiotemporal dynamics of the flame with the heat signatures imposed on the wall. Measurements are performed for lean H2-air flames with equivalence ratio Φ = 0.56 and compared to CH4-air flames with Φ = 1. Twall signatures for H2-air Φ = 0.56 exhibit alternating high and low-temperature vertical streaks associated with finger-like flame structures, while CH4-air flames exhibit larger scale wrinkling with identifiable crest/cusp regions that exhibit higher/lower wall temperatures, respectively. The underlying differences in flame morphology and Twall distributions observed between the CH4-air and lean H2-air mixtures are attributed to the differences in their Lewis number (CH4-air Φ = 1: Le = 0.94; H2-air Φ = 0.56: Le = 0.39). Findings are presented at two different passage spacings to study the increased wall heat loss with larger surface-area-to-volume ratios. Additional experiments are conducted for H2-air mixtures with Φ = 0.45, where flame propagation was slower and was more suitable to resolve the wall heat signatures associated with thermodiffusive instabilities. These unstable flame features impose similar wall heat fluxes as flames with 2–3 times greater flame power. In this study, these flame instabilities occur within a small space/time domain, but demonstrate the capability to impose appreciable heat fluxes on surfaces.

Boiling heat transfer characteristics of distributed jet array impingement on metal foam covers with different wettability

Wettability may have significant influence on jet impingement boiling on metal foam, but the effect mechanism of metal foam wettability remains unclear. In this study, the boiling heat transfer characteristics of distributed jet array impingement on hydrophobic and hydrophilic metal foam covers were experimentally researched and compared with those on uncoated metal foam covers to analyze the influence of wettability. The experimental conditions cover contact angles of 14.0–158.7°, pore densities of 20–40 PPI, porosities of 92 %-97 %, thicknesses of 3.0–5.0 mm, and jet velocities of 0.5–4.0 m·s−1. The results show that, the obtained maximum heat flux and maximum heat transfer coefficient are up to 538.1 W cm−2 and 57.9 Kw m−2 K−1, respectively; the hydrophobic metal foam cover has a 4.8 K lower surface superheated degree at the onset of nucleate boiling, but a 7.5 % lower maximum heat transfer coefficient compared with the uncoated one; the hydrophilic metal foam cover shows a less deterioration after the departure from nucleate boiling but a 5.3 K higher surface superheated degree at the onset of nucleate boiling than those of the uncoated one. A new correlation for boiling heat transfer coefficients was developed with a mean relative error of 9.75 %.

Investigation on dynamic heat transfer characteristics and fin geometric parameters in latent heat storage system with vertical tubes and longitudinal fins

Latent heat storage plays an important role in the utilization of solar energy. However, the low thermal conductivity of phase change materials (PCM) significantly reduces the heat transfer efficiency of latent heat storage systems. To enhance its storage/release efficiency, optimizing the fin geometry is essential. This paper establishes a validated three-dimensional numerical model that considers PCM natural convection to study the effects of fin height and number on the heat transfer process. The fin volume of all models is kept constant, and the fin height is determined by the annular space. The impact of fin heights (0.3ΔR, 0.5ΔR, 0.7ΔR, 0.9ΔR) and numbers (4, 8, 10, 16) on heat transfer efficiency was investigated by analyzing the PCM temperature distribution on the shell section, the liquid fraction within the shell over time, and the average heat transfer rate and heat flux. The results show that increasing the fin height from 0.3ΔR to 0.9ΔR reduces the heat storage and release completion times by 61.16% and 45.43%, respectively. Similarly, increasing the number of fins from 4 to 16 reduces the heat storage and release completion times by 33.35% and 31.13%, respectively. The study concludes that increasing both the fin number and height dilutes the heat flux between the fin and PCM during both the heat storage and release processes, with fin number having a more significant effect on reducing heat flux than fin height. Therefore, when the fin volume remain constant, increasing fin height is more conducive to improving the heat transfer performance of the PCM. These findings will provide a foundation for the application of finned tube energy storage systems in building energy conservation and other fields.

Effect of nanofluid sedimentation on heat transfer and critical heat flux in boiling flows

Effect of nanofluid sedimentation on heat transfer and critical heat flux in boiling flows

Experimental and numerical study of a novel composite building wall U-value

The present study aims to minimize the overall heat transfer coefficient (U-value) and heat flux through building walls to improve indoor thermal comfort, using different bricks and insulating materials in building wall construction. The building walls of three different types of bricks (Type-1, Type-2, and Type-3) were tested to compare the U-value through the wall. The brick with the low U-value was used to build a composite wall with the addition of glass wool and air cavity. The experiment was conducted maintaining different hot chamber temperatures (40℃ to 65℃) to see the effect of hot chamber temperature on the U-value. The temperature of the cold fluid chamber is increased by 0.9℃ in 0 to 210 min of testing duration. The maximum heat flux using Type-3 brick in the composite wall is reduced by 3.37%, and the percentage reduction in indoor temperature is 40.83%. Steady-state numerical analysis was also performed to perceive the temperature distribution on the composite wall surface and found to be well-matched with the experimental model with a percentage deviation of 1.96%. The results suggested that using glass wool and air cavities with low thermal conductivity, low density, and high specific heat capacity effectively reduces the U-value throughout the building wall.