Convective Coefficient Research Articles

The effect of baffles on vertical wall PCM to enhance natural convection

The objective of this study is to enhance convective heat transfer within phase change materials (PCM) using baffles on a vertical wall. PCMs have major drawbacks of low thermal conductivity, high viscosity, and limited natural convection. To address these issues, we investigated a passive approach to enhance natural convection with baffles. The new approach is to arrange multiple baffles with gaps on the opposite side of the heated vertical wall. This placement can minimize vertical temperature difference by blending upward-flowing hot liquid PCM with downward-flowing cold liquid PCM across the gaps and also increase circulating velocity by shortening path of circulation cell through sections divided by baffles. Visualization experiments of shadowgraphy and particle image velocimetry (PIV) were conducted to compare the case with baffle (WB) against the case without baffle (NB). The presence of the baffles lowered the average temperature difference in the horizontal direction by 40%. The WB case reduced the temperature difference in the vertical direction by 50%, compared with the NB case. Furthermore, comparison of the WB with NB showed that at the heat power levels of (4, 6, and 8) W, the convection coefficient increased by up to (28, 34, and 27)%, respectively. Therefore, by changing the internal flow pattern, the baffles on the vertical wall improved convective heat transfer performance.

Experimental study on the influence of restricted distance on the discrete flame spread of different widths

In this paper, the effects of varying material widths on the propagation of discrete fires within diverse confined space conditions are investigated experimentally. The width-induced mechanisms influencing the flame spread of discrete solid surfaces in distinct confined spaces are comprehensively discussed, encompassing flame characteristics, pyrolysis front dynamics, flame spread rate, and mass loss rate. The findings reveal that the dimensionless flame height and width conform to a negative power relationship at varying restricted distances. Furthermore, a positive correlation between the width of the PMMA in confined spaces and both the flame spread rate and the mass loss rate is established. Notably, when the width is fixed, the confined space plays a dominant role in the propagation of flames. As the restricted distance decreases, the flames become more slender. Additionally, flame height, flame spread rate, and mass loss rate increase with the widening of the material. However, as the restricted distance becomes smaller, the effect of width becomes less pronounced. Under conditions of same restricted distance, an increased width results in a higher radiation factor, thereby enhancing the thermal feedback of the wall to the material, which in turn accelerates the spread rate of discrete flames. Moreover, due to the dominant role of radiant heat feedback provided by the confined walls to the discrete PMMA panels, the proportion of heat transferred by convection in the flames is minimal. Consequently, the influence of the convective coefficient's width variation on flame spread is relatively minor within confined spaces. Ultimately, an energy conservation model under confined spaces was established, which facilitated the analysis of the comprehensive mechanism of flame spread influenced by varying confined spaces and material widths.

FLOW AND HEAT-TRANSFER CHARACTERISTICS IN SMALL-DIAMETER TUBE BUNDLES WITH A STAGGERED LAYOUT: AN EXPERIMENTAL STUDY

While past research has primarily focused on heat transfer in larger diameter tubes, the capabilities of small-diameter tubes have often been estimated using correction factors. However, with the evolution of compact heat exchangers, conducting dedicated studies on small-diameter tube bundles has become increasingly crucial to achieve more precise conclusions. An experimental research on flow and heat-transfer characteristics of staggered tube bundles with different tube diameters (2, 3, and 5 mm) is conducted. In the experiment, the number of rows (4-12), the mass rate of the air (0.06-0.18 kg/s), and the transverse tube pitch (<i>S</i><sub>1</sub>/<i>d</i> = 2, <i>S</i><sub>1</sub>/<i>d</i> = 3) are variables to study the characteristics of the airside flow resistance and heat transfer. The three main conclusions of the experimental results are as follows: (1) Under the same conditions, the smaller tube diameter leads to the larger airside convective heat-transfer coefficient. Besides, the deviation between the Nusselt number of the experiment and the empirical correlation of Žukauskas is in the range between -14 and -10%; (2) The effect of transverse distance on heat transfer is not obvious, but the convective heat-transfer coefficient increases significantly with the increase of row number; (3) The external pressure drop of the tube exhibits an exponential increase with the air-flow rate. Particularly in the experimental samples with smaller diameters, the outflow resistance of the tube is noticeably higher compared to other tubes. Finally, new empirical correlations of the airside convection heat transfer for the small-diameter staggered tube bundles are fitted according to the experimental data, and it is hoped to provide a reference for the more accurate design of tube-bundle heat exchangers.

Experimental and numerical modeling of the heat generation characteristics of lithium iron phosphate battery under nail penetration

This study conducted nail penetration tests on 20 Ah prismatic LiFePO4 batteries and simulated the slow release of Joule heat and side reaction heat by combining a new thermal model with a parameter optimization method. The results indicate that the 50% and 80% SOC LiFePO4 batteries only release Joule heat under penetration, while the side reaction heat is acquired under 100% SOC besides Joule heat. Moreover, approximately 56.4% of the stored electrical energy is converted into Joule heat, which accounts for the majority of the total heat production of 100% SOC LiFePO4 battery under penetration, while side reaction heat accounts for only 6.4%. Furthermore, the exothermic side reactions of 100% SOC LiFePO4 battery under penetration can be effectively suppressed when the electrical energy release ratio is less than 0.52, or the convection coefficient between the battery and its surroundings exceeds 12 W/m2K. This numerical study expands the analysis of the heat generation characteristics of LiFePO4 batteries during penetration and provides practical guidance for system safety design.

Effect of electrolyte concentration on bubble evolution and mass transfer characteristics on surface of photoelectrode

In the photoelectrochemical water splitting reaction system, bubbles will cover the reaction area on the photoelectrode surface, affecting the reaction impedance and gas-liquid mass transfer. A laser irradiation system is built and it is coupled with an electrochemical workstation and high-speed microscopic imaging system. The evolution behavior and mass transfer characteristics of single O<sub>2</sub> bubble on the TiO<sub>2</sub> photoelectrode are studied at different electrolyte concentrations (Na<sub>2</sub>SO<sub>4</sub>, 0.1–2.0 mol/L). With the increase of electrolyte concentration from 0.1 mol/L to 2.0 mol/L, the solution resistance and bubble additional resistance decrease, and the overpotential in the stable growth stage of bubble decreases from 0.113 V to –0.089 V. The bubble will cause the fluctuation of overpotential in the nucleation, growth and detachment stages, which is consistent with the impedance change caused by the change of dissolved oxygen concentration in the liquid phase. By analyzing the correlation between gas evolution efficiency and bubble coverage, it is found that the increase of electrolyte concentration will lead the bubble coverage and gas evolution efficiency to decrease simultaneously. By calculating the Sherwood dimensionless number, the results show that the total convective mass transfer coefficient increases with the electrolyte concentration increasing. Single-phase natural convection plays a dominant role in the process of gas product transfer, and its mass transfer coefficient is one order of magnitude larger than that of bubble-induced convection. In summary, by adjusting the electrolyte concentration, the bubble on the gas evolution photoelectrode surface can be effectively removed and the mass transfer of the system can be optimized, which is of great significance in improving the efficiency of photoelectrochemical water splitting.

An almost second order uniformly convergent method for a two-parameter singularly perturbed problem with a discontinuous convection coefficient and source term

<p>In this paper, we discuss a higher-order convergent numerical method for a two-parameter singularly perturbed differential equation with a discontinuous convection coefficient and a discontinuous source term. The presence of perturbation parameters generates boundary layers, and the discontinuous terms produce interior layers on both sides of the discontinuity. In order to obtain a higher-order convergent solution, a hybrid monotone finite difference scheme is constructed on a piecewise uniform Shishkin mesh, which is adapted inside the boundary and interior layers. On this mesh (including the point of discontinuity), the present method is almost second-order parameter-uniform convergent. The current scheme is compared with the standard upwind scheme, which is used at the point of discontinuity. The numerical experiments based on the proposed scheme show higher-order (almost second-order) accuracy compared to the standard upwind scheme, which provides almost first-order parameter-uniform convergence.</p>

Numerical Simulation of Heat Transfer through Uniform Multilayer Walls using ANSYS

The paper presents a study on modelling the main heat transfer parameters through opaque building elements. The need for models to assess the thermal behavior of a building has increased greatly in recent years, primarily to allow a more accurate determination of the energy consumption of buildings to evaluate the performance of heating systems. In recent times, mathematical models and software have been implemented to obtain simulations that are very close to the real functioning of the main components of buildings. The present article consists in modeling a multi-layer homogeneous wall, which separates the interior space of an enclosure (rooms) from the outside environment. The article aims to carry out a series of simulations on the structure of a non-homogeneous multilayer wall using the ANSYS program. The simulations have been performed highlight how the values vary (both numerically and graphically) for a series of characteristic parameters such as heat fluxes, temperatures, convection coefficients. The values of the parameters obtained with the ANSYS program were also compared with those obtained by classical numerical calculations.

NUMERICAL MODELING OF THE ASYMMETRIC HOT PLATE METHOD CONSIDERING LATERAL CONVECTIVE LOSSES

In this study, a numerical model of the asymmetric hot plate method was developed to determine the thermal properties of materials. The proposed model considers the convective heat loss around the device. First, a numerical calculation code is developed to determine the temperature evolution at the measurement points. This calculation code is then used to perform a reduced sensitivity study on various parameters. The Levenberg-Marquardt algorithm was used to identify the thermal properties of different materials from the thermogram. The impacts of the convection coefficient h and insulation thickness on the calculation of the estimates were studied. Finally, an experimental study was conducted using several materials. The thermal properties were compared with experimental results obtained using other measurement methods. A satisfactory agreement was obtained.

Numerical Study of the Effect of the Ejection Angle on the Cooling Performance of a High-Pressure Gas Turbine

In modern gas turbines, the purge flow plays an important role in improving the performance of the gas turbine, as it is adapted against the problem of the ingestion of hot gases in the inter-disc cavity of the turbine. The performance of this purge flow is necessarily influenced by some geometrical and physical parameters. This paper presents a 3D numerical study on the influence of the ejection angle on the cooling performance in a high pressure transonic gas turbine with the presence of the cavity. Five inclination cavity angles are compared (𝝓 = 30°, 45°,60°,75°and 90°) by using ANSYS Fluent, a steady flow Reynolds average Navier-Stokes equations are solved, and the turbulence model k-ω SST has been chosen based on comparison of the distribution of velocity and heat transfer convective coefficient with experimental results in a previous work. It was seen that that cooling is limited to the suction side because of the non-uniform pressure distribution of the main flow and the high pressure of the stagnation point. The optimal configuration was determined for an ejection angle of 30°.

Stability Evaluation of Nanofluids Suitable for Enhanced Oil Recovery

The evaluation of metal oxides nanoparticles stability in the base fluids has become a major aspect in enhanced oil recovery process in recent years. Physical and chemical properties of ZnO nanofluids have caught attention of many researchers because they are easily dispersed in the base fluids, better convective coefficient of heat transfer, and wider bandgap which make them remarkable nanofluids candidate compared to other metal oxides. In this study, the stability of nanofluid of zinc oxide nanoparticles was evaluated using different stability tests and analysis. However, ZnO nanoparticles were synthesized using sol-gel method and nanofluids of different concentrations were prepared. The prepared nanoparticles were characterized using various characterization techniques such as XRD, FTIR, TEM, and Zeta sizer. The stability of prepared nanofluids was investigated using sedimentation test, UV-vis spectrophotometer, and zeta analyzer. Based on physical investigation and UV-vis spectra observations, ZnO nanoparticles in base fluids have shown a good colloidal stability in addition to their high zeta potential values of-43.7 (mV) after seven days which makes it a successful potential candidate for enhanced oil recovery applications.

Comparative summer thermal performance analysis between open ventilated facade and modular living wall

In recent years, passive solutions for building envelopes have become much more common due to their capacity to decrease the heat flux through the envelope during summer time. Vertical greenery systems (VGS) are emerging as an interesting method of decreasing the thermal demand of cities, and also improving the quality of urban life. Open ventilated facades (OVF) have gained popularity due to their capacity to enhance the thermal resistance of the building envelope. As part of a project carried out in a Paslink cell in Vitoria-Gasteiz, an experimental campaign with full-scale VGS and OVF was carried out during the summer season to assess the thermal performance of a modular living wall (MLW) with respect to an OVF. The objective is to demonstrate that a stochastic differential equations (SDE) model can be used to assess the cooling requirements of an MLW and an OVF. An analysis was carried out to evaluate how different characteristics of the main facade affect performance, such as thermal resistance, solar absorption coefficient and convection coefficient. The results of these experiments show that both MLW (46 %) and OVF (67 %) configurations significantly minimize solar heat loads compared to non-passive bare wall (BW) facades, which are the reference configurations.

The effect of characteristics of the mold with and without pin fins on the cooling profile in rotomolding sistem by FEM

In the rotomolding processes, the cooling stage is important, which affects significantly the productivity and quality of the final product. Therefore, in this work, a computer simulation was performed, studying the effect of the pin fins on the mold in the cooling phase in the rotomolding system, thus, steel and aluminum molds with and without pin fins were considered. In this work we had the following results, according to the optimization process, due to the high conductivity and low density of aluminum, it was verified that less pin fins were needed around the mold aluminum than for steel, but the length of the pin fins was greater for aluminum; in addition, the mold with fins had a faster cooling than the mold without fins; meanwhile, the use of appropriately sized fins significantly reduces the cooling time, nevertheless, as has been observed that the increasing or decreasing the fin number, the cooling profile is very little affected. However, the variation of the conviction coefficient applied to the external surfaces was also investigated, but the behavior of the fins are more efficient when the convection coefficient is higher so improving productivity. Finally, he characteristic of the mold material was also examined, as the steel mold with fins has a sharper cooling compared to the same material without fins, however, the aluminum was more efficient, because the cooling process was faster in molds of aluminum with fins. Emphasizing, the numerical simulation can be satisfactorily applied in the industrial process of rotomolding, of a simplified manner and qualitatively, promoting growth in production and reducing so the costs considerably associated with the production.

An Investigation on Constant Convection Coefficient Assumption in Decoupled Conjugate Heat Transfer

An Investigation on Constant Convection Coefficient Assumption in Decoupled Conjugate Heat Transfer

Calculation of convection coefficient during steel hardening of axisymmetric probes in thermal oils

The paper presents the results of the convection coefficient during the process of two-dimensional axisymmetric steel hardening of in two different thermal oils used in the area of thermal treatment of steel. One thermal oil was Isorapid 277 HM preheated to the set temperature, and the other one was Marquench 722 also preheated to the set temperature. The experimental work setup consists of six quenches, three dimensionally different axisymmetric probes in two different thermal oils. Steel hardening was done under strictly controlled conditions: heating the sample to the tempering temperature of most steels in a nitrogen atmosphere, vertical slow immersion of the test sample in the opposite flow of thermal oil. The temperature of the thermal oil was kept constant, stopping the test sample in the sample steel hardening area and non-stationary data collection until thermal oil equilibrium temperature with interruption of data collection. The data collection rate was two probes/second in parallel from four points located within the test cylinder. Three measuring points were located directly below the surface of the cylinder distributed along the height of the cylinder, while the fourth one was in the centre of gravity of the cylinder. Time-measured temperature values in the indicated four points of each probe were written in a text file. The set mathematical model describes the non-linear two-dimensional axisymmetric non-stationary heat transfer through the cylinder. The set mathematical model requires the inverse solution of the heat conduction problem, that is, ill-conditioned problems, so the solution of the problem came down to a sufficiently accurate estimation of the unknown convection coefficient imposed on the outer surface of the probe. The chosen solution algorithm was the one from previously published works. The solution algorithm was based on a iterative combination of the finite element method (FEM), while the second part of the solution algorithm is the Levenberg Marquardt method (LMM) from the line of optimization deterministic algorithms, which takes the calculation results of the temperatures at the points of the computational domain from the FEM, and for comparison, that is, error minimization, using the experimental results of temperature measurements at the same place.

How to design the drying of porous materials?

The convective drying of porous materials is studied at the pore scale using a two-component two-phase Lattice Boltzmann model. Capillary pumping from coarse to fine pores is found to enhance the drying in first drying period at high rate. A ‘toy’ model of four connected channels is developed to design the drying of porous materials. This approach has to be seen to supplement the continuum modelling of drying using known moisture transport properties and convective moisture transfer coefficient, allowing to better understand convective drying and design the pore structure to optimize the drying rate.

Quantification of liquid nitrogen injection efficiency and phase change expansion boost laws in coal seam borehole

The implementation of liquid nitrogen injection into coal seam boreholes is a prerequisite for its application to increase coal seam permeation. Exploring the efficiency of liquid nitrogen injection is of great significance in optimizing the settings of key injection parameters, thereby reducing costs. By using high-pressure liquid nitrogen tank, flowmeter, endoscope, thermocouple and mass balance, experiments of different liquid nitrogen injection rates were carried out in 1 m long coal seam borehole. The rules for changing the liquid nitrogen net level in the borehole at different injection rates were obtained: slow growth-linearly growth-slowly growth to stability, and the maximum net liquid level could reach 60 cm. The effective convection coefficient and radiation coefficient during the transient film boiling at the early stages of liquid nitrogen injection were calculated, and it was found that convective heat transfer dominated this stage. The liquid nitrogen injection efficiency at different injection rates was quantitatively investigated, and the maximum efficiency was 8.75 %, and the injection efficiency increases with the increase of injection rate. After stopping the injection, the net liquid nitrogen level in the borehole was analyzed, and they all decreased exponentially. A liquid nitrogen phase change expansion boost model was established and experimental verification passed. Further, it was calculated that the maximum value of liquid nitrogen phase change expansion pressure in a closed borehole of 1 m long coal seam was 45.8 MPa. The expansion pressure can play a dominant role in fracture propagation during the fracturing process, and combined with the damaging effects of liquid nitrogen cold shock on coal, it has a great development prospect in the field of coal seam permeability enhancement.

Two-phase flow instability characteristics of HTGR Once Through Steam Generators

High Temperature Gas-cooled Reactors (HTGR) have the advantage of inherent safety compared with most Gen III reactors. They also have very high temperature, which make the plant have high electricity generation efficiency and have has the potential for hydrogen production. Once Through Steam Generator (OTSG) is responsible for transferring high temperature heat from primary loop helium to secondary loop composed of water and steam. The evaporation tubes of HTGR OTSG are very long due to the low heat transfer coefficient of helium convection over tube bundles. A Two-phase flow instability may occur in HTGR OTSG, which can cause pulsation of the mass flow rate, system pressure and temperature. This will not only interfere with the control system, but also cause thermal fatigue damage in the structures. Thus, it is important to predict and avoid two-phase flow instability in HTGR OTSG. Three types of flow instabilities are investigated for HTGR OTSG using both a linear analytical model and a nonlinear numerical model. They are flow excursion (or Ledinegg instability), density wave oscillation and pressure drop oscillation. For the nonlinear numerical model, RELAP5 software is adopted. Flow excursion and pressure drop oscillation are discussed based on the predicted hydraulic characteristic curve of HTGR OTSG (pressure drop vs. mass flow rate curve). Density wave oscillation is investigated using the transient variation of the outlet mass flow rate. For the linear analytical model, a set of ordinary differential equations are established using integration and small perturbation methods. The variables of differential equations are the length of the subcooling water region, the length of both the subcooling water and saturated two-phase regions, the average density of the saturated two-phase region and the inlet mass flux. The stability of OTSG is investigated using the eigenvalues of the coefficient matrix of the ordinary differential equations. Based on the results calculated using the linear analytical model and nonlinear numerical model, there is no flow excursion, pressure drop oscillation or density wave oscillation at the designed partial and full power levels (30%, 50%, 75% and 100%). Thus, HTGR OTSG can operate stably and have a sufficient safety margin at the designed power levels. Density wave oscillation will happen when decreasing system pressure. However, increasing inlet resistance coefficient can avoid density wave oscillation effectively. The tube lengths’ influences on the stability boundary of OTSG are also investigated and it shows no strong dependency.

Effect of numerous plate holes in a cooling tower on heat transfer optimization

An industry requires a cooling medium to reduce heat in an industrial machine during operation. Companies generally use cooling towers for engine cooling media. The common issue is that heat reduction and heat transfer rate are not significant. Therefore, a new variation is needed to ensure that the cooling tower can effectively lower the temperature of the machinery. The problem statement aims to determine the parameters that can enhance both the heat transfer rate and the heat transfer coefficient in cooling towers. The objective is to determine the heat transfer rate and heat transfer coefficient. The method used is experimental by varying the water inlet in five variations of the cooling tower plates, they are being 48, 60, 80, 120, and 250 holes. The results showed that the highest temperature difference occurs at Tin 80oC with the variation of 250 holes, which is 9.34oC, and the highest heat transfer value reached 1833.17 watts. Meanwhile, the lowest temperature difference occurred at Tin 60oC with a variation of 48 holes, which is 3,98oC, and the lowest heat transfer value reached 787.47 watts. The highest convection coefficient occurs at Tin 70oC with the variation of 250 holes, which is 117.74 W/m²⋅K. The lowest convection coefficient occurs at Tin 80oC with a variation of 48 holes, which is 77.36 W/m²⋅K. This can be concluded that the temperature difference (⧍T), heat transfer rate, and heat transfer coefficient will increase when the number of holes in each plate variation increases.

Thermal Hydraulics Simulation of a Water Spray System for a Cooling Fluid Catalytic Cracking (FCC) Regenerator

Olefins are crucial building blocks for petrochemical industry, serving as raw materials for the production of various products such as plastics, synthetic fibers, detergents, solvents, and other chemicals. In FCC, heavy petroleum feedstocks are injected into a catalytic cracking unit, where they are mixed with a catalyst. The catalyst aids in breaking down the large hydrocarbon molecules into smaller fragments, including olefins like Propylene and Ethylene. These polymerization reactions occur at high temperatures. They demand that heat removal occurs as quickly as possible in order to control the reactor temperature and to avoid “hot spots” in the Regenerator or localized oxidation reactions (and to avoid creep rupture of the regenerator steel cladding). The cooling of the regenerator cladding surface can be achieved by impinging water droplets (spray), ejected from a spray nozzle. Spray cooling can provide uniform cooling and handle high heat fluxes in both a single phase and two phases. This research provides a thermal hydraulic design of regenerator spray cooling systems. In the framework of this research, Fire Dynamics Simulator (FDS) software was applied in order to simulate the temperature field and the water vapor mass fraction. A COMSOL Multiphysics finite element code was used in order to calculate the temperature field inside the regenerator cladding. The calculated surface temperatures and heat transfer convective coefficient, obtained using FDS software, were validated successfully against COMSOL numerical results and previous results in the literature. The numerical simulations were carried out for two cases. The first case was carried out at a distance of 0.5 m, and the second case was carried out at a distance of 0.2 m. A grid sensitivity study was carried out on the FDS model. Numerical integrations were carried out over time in order to calculate the average temperatures. The difference between these four average temperatures, calculated by applying different grids, is less than 7.4%. The calculated surface temperatures and heat transfer convective coefficient were validated successfully against COMSOL numerical results and previous research. It was shown that the calculated temperatures decrease in the second case. The water spray system managed to cool the steel wall more effectively as the water spray system approaches the steel cladding.

Thermal characterization of carbon-rich boron carbonitride foams for thermal management applications

The increasing need to evacuate heat from electronic components encourages the use of new thermal materials. Motivated by the good heat convection of graphene foam (GF), we have integrated boron and nitrogen atoms into GF, thereby obtaining carbon-rich boron carbonitride (BCN) foam, which shows a significant improvement to its thermal characteristics. We used the infrared micro-thermography method and showed that carbon-rich BCN foam has an exceptionally large effective convection area and a high convection coefficient, which is comparable to that obtained under forced convection. Additionally, carbon-rich BCN foam presents remarkable thermal stability with a degradation temperature of ∼750 °C. Finally, we attached the carbon-rich BCN to an electric circuit and demonstrated its feasibility to evacuate heat from electronic components. Therefore, we pave the path for the integration of carbon-rich BCN foams in advanced thermal applications, such as thermal management reinforcement in composites and heat dissipation elements.