Global Heat Transfer Coefficient Research Articles

Critical role of transport phenomena in the performance of a catalytic fixed bed reactor for the CO2 hydrogenation to CH4 evaluated by modeling and simulation

CO2 hydrogenation with renewable hydrogen enables the production of CH4 to be used as energy carrier (CO2 + 4H2 ⇆ CH4 + 2H2O). Steady state 1D pseudo-homogeneous and heterogeneous models, in the absence or presence of mass and energy transport in the axial direction of the reactor and inside the solid phase, are considered to simulate a non-isothermal fix bed reactor. Numerical strategies for solving these models in Matlab R2022b are discussed. The pseudo-homogeneous plug-flow model is not able to describe the experimental results irrespective of the global heat transfer coefficient considered. The inclusion of mass and energy transport in the reactor axial direction is crucial for describing the experimental data. The inclusion of mass and heat transport inside the solid phase by the heterogeneous model enables the best fitting. This work provides a valuable modeling tool for future industrial-scale reactor design.

Comparative study of the effectiveness of time series and arima modeling x RNA approach in a Multi Layer Perceptron (MLP) model in predicting fouled in petroleum pre-processing heat exchangers

This work presents a comparison between two approaches based on Artificial Intelligence techniques to predict critical deposition in the battery of shell and tube heat exchangers, used in petroleum pre-processing. Deposition or incrustation, which occurs on the thermal exchange surfaces of heat exchangers, during operation, can reduce the efficiency of this equipment and generate an increase in the cost of refining for the petrochemical plant due to the modification of ideal operating parameters and changes. process variables, such as flow, pressure and temperature, input and output of heat exchangers, as well as thermodynamic parameters, such as fouling resistance coefficient and global heat transfer coefficient. The proposed methods for comparing results used the analysis and prediction of time series based mainly on integrated autoregressive moving average (ARIMA) and Deep Learning models and, on the other hand, an artificial neural network (ANN) model based on the Multi Layer architecture. Perceptron (MLP) with the Backpropagation error backpropagation algorithm. The data was collected in exchangers of a petroleum preheating battery in the years 2014 to 2021. The general objective of this work is to present the results of both approaches and compare, since they aim to predict the deposition in the heat exchanger of the shell and tube type, thus helping to minimize maintenance costs and increase the energy efficiency of the petrochemical plant, making operations safer and more efficient, bringing significant benefits to the petroleum industry.

Modeling of finned flat tube heat exchangers and search of Nusselt-Reynolds numbers correlations

Using a wind tunnel, a series of model finned flat tube copper radiators was studied. All geometrical parameters of the studied radiators were fixed except for the height of the fin, which was varied in the range from 5 to 20 mm. It was found that the global water-air heat transfer coefficient weakly depends on the water flow rate and significantly depends on the air flow rate. Correlations between the Nusselt and Reynolds numbers for fins with different heights are found. Based on the obtained correlations, a procedure for scaling the heat exchanger, verified experimentally, is proposed. The experiments performed demonstrate the efficiency and time-saving potential of the proposed method for choosing the optimal finned flat tube heat exchanger for adsorption heat conversion systems.

The thermophysical properties of a promising composite adsorbent based on multi-wall carbon nanotubes for heat storage

AbstractThe use of energy from alternative energy sources as well as the use of waste heat are key elements of an efficient energetics. Adsorption heat storage is a technology that allows solving such problems. For the successful operation of an adsorption heat accumulator, it is necessary to analyze the thermophysical characteristics of the system under the conditions of the operating cycle: heat transfer coefficient adsorbent-metal (α2), overall (U) and global (UA) heat transfer coefficients of heat exchanger. Multi-walled carbon nanotube (MWCNT) composites are very promising for adsorption-based renewable energy storage and conversion technologies. In this work at the stage of heat release, α2 was measured by the large pressure jump (LPJ) method, at the stage of heat storage by large temperature jump method (LTJ), which made it possible to obtain thermophysical characteristics that corresponded to the implementation of the real working cycle as much as possible. The heat transfer coefficients for a pair of adsorbent LiCl/MWCNT—methanol are measured for the first time under the conditions of a daily heat storage cycle both at the sorption stage (α2 = 190 W/m2K) and at the desorption stage (α2 = 170 W/m2K).

Metal foam recuperators on micro gas turbines: Multi-objective optimisation of efficiency, power and weight

Small size and high efficiency of micro gas turbines require a higher surface-to-volume ratio of recuperators. Conventional recuperators can achieve a range of 250–3600 m2/m3. Advances in materials and manufacturing, such as metal foams, can increase significantly the exchange surface and improve compactness ranging approximately from 500 to over 10,000 m2/m3, due to their exceptional micro geometry. The main advantage is that the increase of surface area does not impact the cost of the heat exchanger as much as conventional recuperators due to their easy manufacturing. This work addresses the optimisation of the recuperator using multiple objectives satisfying efficiency, power output and weight criteria, offering a holistic approach that takes into account the entire system rather than individual components or channels. A model is developed to represent the performance of a compact heat exchanger in micro gas turbines. The recuperator is an annular heat exchanger with involute profile filled with porous media in a counterflow arrangement on the hot and cold sides. The model allows the evaluation of the effect of the recuperator geometry features on the electrical efficiency, power output and weight savings in a micro gas turbine. Existing models for the global heat transfer coefficient, effective thermal conductivity, surface area and pressure drop of porous media are selected and implemented. The design variables of multi-objective are the pore density, porosity and number of channels, whilst the objectives are the overall electrical efficiency, power output and recuperator weight. The problem is solved using the Non-Dominated Sorting Genetic Algorithm (NSGA-II) to determine an approximation of the Pareto front, whilst the accuracy of the approximation is assessed against the solution obtained by an exhaustive search. The comparison shows that NSGA-II outperforms an exhaustive search by at least 90 % in terms of computational efficiency. These results allow the quantification of the impact of metal foam technology on performance metrics of the recuperator as well as the entire system. This quantitative analysis provides valuable insights into the behaviour of metal foam recuperators in micro gas turbines. An optimal design with 30 % efficiency and 28 kW power output appears in pore densities of approximately 10 and 20 pores per inch (PPI) for the air and gas side respectively, and a porosity of 85 %, which leads to a state-of-the-art recuperator weight of 48 kg. The efficiency improvement over the industry standard is 15 %, with only a 2.5 % reduction in power output.

Accuracy in evaluating convective heat transfer coefficient by RANS CFD simulations in a rectangular channel with high aspect ratio and 60° tilted staggered ribs

A numerical analysis of heat transfer characteristics of an air flow through a narrow rectangular channel of 1:10 aspect ratio with ribbed surfaces has been carried out, by means of a CFD commercial code, exploiting Reynolds Averaged Navier-Stokes (RANS) equations. The channel is 120 mm wide, 840 mm long, with 60° tilted staggered ribs. Ribs have a square cross section with two different side heights (2 mm and 4 mm) and three different values of dimensionless pitch (10, 20 and 40). The numerical results have been compared with experimental data obtained by the authors inside a ribbed channel both with same geometry and operating conditions as one numerically modelled. Agreement between numerical and experimental data on convective global heat transfer coefficient, i.e., averaged over the whole channel, is discussed for the six different configurations considered. Moreover, performances are presented by considering at the same time both heat transfer enhancement and pressure-drop penalization, highlighting strengths and weaknesses per each configuration. This work is aimed at finding a suitable configuration of a CFD model with RANS that will allow authors to apply it to the range of dimensionless pitches and side heights during early design-phases of parametric studies.

The Heat Transfer in Plate Fin Heat Exchanger for Adsorption Energy Storage: Theoretical Estimation and Experimental Verification of the Methodology for Heat Accumulation Process

Adsorption energy storage is a promising resource-saving technology that allows the rational use of alternative heat sources. One of the most important parts of the adsorption heat accumulator is the adsorber heat exchanger. The parameters of heat transfer in this unit determine how fast heat from an alternative energy source, such as the Sun, will be stored. For the design of adsorption heat accumulators, plate fin heat exchangers are mainly used. In this paper, the procedure for the estimation of the global heat transfer coefficient for the adsorber heat exchanger depending on its geometry is considered. The heat transfer coefficient for a LiCl/SiO2 sorbent flat layer under conditions of heat storage stage was measured. Based on these data, the global heat transfer coefficients for a number of industrial heat exchangers were theoretically estimated and experimentally measured for the adsorption cycle of daily heat storage. It was shown that theoretically obtained values are in good agreement with the values of the global heat transfer coefficients measured experimentally. Thus, the considered technique makes it possible to determine the most promising geometry of the plate fin heat exchanger for a given adsorption heat storage cycle without complicated experiments.

Experimental and theoretical thermal investigation of bio-composite panels based on sawdust particles

This work focuses on the valorization of sawdust waste in order to develop a new bio-sourced insulation material. To assess the dynamic thermal behavior of a multi-layer external wall insulated with these panels, their thermal characteristics, the time lag, and the decrement factor, were investigated. The results obtained show that when the density increases, the thermal conductivity, the thermal diffusivity, and the volumetric heat capacity increase while the thermal diffusivity decreases. For a thickness of 20 cm of the composite material, the time lag increases, and the decrement factor decreases with increasing density. Moreover, the results obtained for an external wall show that the decrement factor decreases and the time lag increases with the thickness of the thermal insulation. It was also found that the position of the insulation layer has a considerable effect on thermal inertia properties. The wall with insulation on the outside is the most desirable. In addition, the internal global heat transfer coefficient affects the external wall's time lag and decrement factor, while the external global heat transfer coefficient does not affect its inertia properties.

Accuracy in evaluating heat transfer coefficient by RANS CFD simulations in a rectangular channel with high aspect ratio - Part 2: channel with ribbed walls

In this paper, a Computational Fluid-Dynamics (CFD) analysis on heat transfer characteristic using Reynolds Averaged Navier-Stokes (RANS) equations on a rectangular channel with ribbed surfaces is presented. Several numerical simulations have been carried out on convective heat transfer of an air flow through a rectangular channel of 1:10 aspect ratio, 120 mm wide, 840 mm long, with 90° ribs. Ribs have a square cross section with 4 mm side and three different values of dimensionless pitch, namely, 10, 20 and 40. Results on convective global heat transfer coefficient, i.e., averaged over the whole channel, have been compared to experimental data obtained in a channel with the same geometry and boundary conditions. Agreement between numerical and experimental data is discussed for the three different pitches considered. As expected, comparisons show a decreasing reliability for increasing complexity. The aim of this work is to find a suitable configuration of a CFD model with RANS that will permit authors to apply it to the range of dimensionless pitches.

Study on the heat transfer with regard to an off-grid vending machine having a low impact on the environment

Following the current trend of energy economy, the article’s team designed a vending machine made of sandwich panels, in order to minimize the heat transfer from the environment to the chilled volume. The heat inputs calculation was based on the data provided in the sandwich panels’ technical sheets. In this paper, the real performance in operation was monitored, resulting in a deviation between 2.27 and 5.37% of the theoretical values for the global heat transfer coefficient. Furthermore, the TEWI coefficient (Total Equivalent Warming Impact) was evaluated to highlight the energy efficiency of the equipment, operating with refrigerant R290. Thereby, the authors emphasize the importance of the transition to natural refrigerants.

Thermal bridge assessment at industrial buildings

It is well known that the thermal performance of a building is influenced by the solutions that define the building’s envelope. In the case of industrial buildings, due to the use of different envelope solutions (e.g. sandwich panels, prefabricated concrete panels etc.), the types of thermal bridge details are often different in comparison to those currently found in residential, office and administrative buildings. Thus, several numerical simulations were performed by analysing details of envelope elements identified at industrial buildings, especially the areas prone to intensified heat flow (joint areas, intersections, glazed surfaces, and others.). Therefore, the paper will present the assessment of thermal bridge details for four industrial buildings having various destinations. The effects of two-dimensional heat transfer in envelope elements, such as windows, walls, foundations, roofs and doors, will be evaluated. The impact of the thermal bridges will be measured by verifying to what extent the level of thermal performance of each envelope component meets the thermal resistance requirements, as well as the building envelope ensures the global heat transfer coefficient for each of the studied cases. A series of conclusions and recommendations will be provided for specialists and designers in the building energetics field.

Thermal performance and design analysis of U-tube based vacuum tube solar collectors with and without phase change material for constant hot water generation

Solar water heaters are the most promising technology, and they can be effectively used for hot water generation in cold climatic conditions. The moto of this research is the design and development of two compact vacuum tube solar collectors (VTSCs): (i) modified copper finned U-tube based VTSC filled with PEG6000 as a phase change material (PCM). However, it integrates heat transmission and storage capabilities into a unit (ii) conventional U-tube inserted VTSC without PCM. The developed VTSC with PCM was operated under dual (simultaneous and mid-day charging) modes, and their performance was compared with the conventional system at various flow rates. This study explored the design analysis, working principle, and the outcome of fixed mass flow rates on the system's thermal output. The results display that the daily energy output of the developed collector with PCM is 81.54–89.74 % for simultaneous mode and 78.21–86.52 % for mid-day charging mode. Whereas the daily energy output of conventional solar collector without PCM was 69.31–75.54 % for set mass flow rates. The highest thermal efficiency was around 89.74 % for U-tube inserted VTSC with PCM (simultaneous mode) at a high flow rate (30 LPH). The global heat transfer coefficient (UL) was maximum at a low mass flow rate (10 LPH) for conventional system without PCM, followed by mid-day charging and simultaneously operated VTSC with PCM. After comparing both systems, it is found that using PCM in VTSC improves the daily energy efficiency and increases the operating time for 3–4 more hours than conventional system. This analysis revealed that the developed U-tube inserted VTSC with PCM can substantially supply the hot water demand for domestic applications constantly at night/off sunshine hours.

Numerical Study on the Thermal Insulation of Smart Windows Embedded with Low Thermal Conductivity Materials to Improve the Energy Efficiency of Buildings

The building industry accounts for almost 40% of the world's energy consumption. To reduce the global heat transfer coefficient, sustainable buildings should use highly insulated enclosures. As the building envelope serves as a barrier between the exterior and interior of the building, integration of passive solar design principles in its construction, such as smart windows with low thermal conductivity materials are essential. Smart windows may assist to reduce energy consumption by minimizing heat gain by the building, which able reduce the cooling loads while maintaining the thermal comfort for the building users. This study features smart double-glazed windows filled with low thermal conductive materials which are argon and aerogel to improve window insulation in pursuit of energy efficiency improvement. A numerical model is developed in ANSYS Workbench to evaluate thermal insulation performance of argon-filled and aerogel-filled windows by measuring the indoor surface temperature of the building at three critical times of the day. Newton's Law of Cooling is used to compute the empirical value of the heat transfer across the window to compare and validate the numerical data. This study shows that argon-filled and aerogel-filled window able to reduce the heat transfer across the building up 21% and 59% respectively. Aerogel is proven to resist more heat transfer as compared to argon

Theoretical analysis of the influence of the chevron inclination angle on the thermal performance of a gasket plate heat exchanger

Different models are applied for an experimental and theoretical determination of the thermal and hydraulic performance of gasket plate heat exchanger. One of the relevant aspects of recent works is the influence of the chevron inclination angle between the heat exchanger plates. This work aims to analyse the impact of the chevron inclination angle by applying an effective concept in a sunflower vegetable oil cooler. Comparisons are made with theoretical and experimental results from the literature for works that consider angles of inclination equal to 30◦, 45◦, and 60◦. An analysis model that does not consider the inclination angle as an explicit parameter is included for comparison purposes. In addition to the angle of inclination, two other parameters, the mass flow rate of the cold fluid (water) and the number of plates, are considered crucial for determining and analysing the results. Nusselt number, global heat transfer coefficient, effectiveness, heat transfer rate, and outlet temperatures for hot and cold fluids are presented in a graphical format. The results point to the need to improve models applied to gasket plate heat exchangers concerning the influence of the inclination angle since there are significant differences between those obtained and analysed in this work.

Express Method for Assessing Optimality of Industrial Heat Exchangers for Adsorption Heat Transformation

In this work, four radiators with different core geometries were tested using a wind tunnel. The values of the global heat transfer coefficient (UA = 5 ÷ 65 W/K) were measured depending on the flow of air and water. The obtained UA values correlate well with the data of sorption experiments described in the literature. The found correlations between the Nusselt and Reynolds numbers made it possible to propose an algorithm for ranging commercial air radiators for the use in adsorption heat transformers. It is shown that the use of a wind tunnel can serve as an effective tool for express assessment of the prospects of using air radiators for adsorption heat conversion without destroying radiators or their direct testing in a complex adsorption installation requiring vacuum maintenance.

Heat transfer within dynamically structured bubbling fluidized beds subject to vibration: A two‐fluid modeling study

AbstractBubbling fluidized beds are often used to achieve a uniform particle temperature distribution in industrial processes involving gas and particles. However, the chaotic bubble dynamics pose significant challenges in scale‐up. Recent work (Guo et al., 2021, PNAS 118, e2108647118) has shown that using vibration can structure the bubbling pattern to a highly predictable manner with the characteristic bubble properties independent of system width, opening opportunities to address key issues associated with conventional bubbling fluidized beds. Herein, using two‐fluid modeling simulations, we studied heat transfer characteristics within the dynamically structured bubbling fluidized bed and compared to unstructured bubbling fluidized beds and packed beds. Simulations show that the structured bubbling fluidized bed can achieve the most uniform particle temperature distribution because it can achieve the best particle mixing while maintaining a global heat transfer coefficient similar to that of a freely bubbling fluidized bed.

Modeling the impact of spray drying conditions on some Maillard reaction indicators in nano‐filtered whey

AbstractThe objective of this study was to develop a mathematical model to predict the loss of available lysine and the formation of Maillard reaction (MR) compounds in sweet whey during spray drying process. The heat damage and other spray drying process‐related variables were estimated with the compartment model coupled to the drying kinetics of droplets. The model was adequate to determine the outlet temperature of air and the final moisture content of the product. The model predicted available lysine losses like those found experimentally. However, the model predicted a higher furosine concentration than that found experimentally. The browning index exhibited no significant increase for experimental and calculated results during the spray drying, possibly because the MR did not reach the final stages. The model showed to be comparable to those reported in the literature, with the advantages of being computationally less time‐consuming and providing confidence intervals for the response variables.Practical applicationsThis work presents the integration of several mathematical models to predict the loss of available lysine and the formation of some heat damage indicators during the spray drying of whey. The model contributes to the understanding of relevant variables—such as residence time distribution, global heat transfer coefficient of the dryer, thermal properties of the drying media, glass transition temperature, drying kinetics, drying air inlet, and outlet temperature among others—on the loss of food quality regarding the thermal process. This information could help the decision‐making process that can lead to the optimization of spray drying to obtain food with minimal thermal damage and therefore the maximum amount of available nutrient content.

Implementation of the characteristic equation method in quasi-dynamic simulation of absorption chillers: Modeling, validation and first results

Scheme of the Thermal Compressor of the prototype absorption chiller – A transient Behavior. • A quasi-dynamic approach to simulate an absorption refrigeration system was made; • The modeling was based on the characteristic equation method and thermal capacity; • A sensitivity analysis to verify the dynamic behavior of the chiller was conducted; • The behavior results by chillers with different solutions showed the same trend; • The comparison results showed good agreement with errors lower than 5%; This work presents a quasi -dynamic approach to an absorption refrigeration system that uses NH 3 /LiNO 3 as the working fluid. The goal is linked to the development and validation of a mathematical model, based on the characteristic equation method, to simulate the quasi -dynamic behavior of absorption chillers. The mathematical model was developed using the first law of thermodynamics through the conservation of mass and energy and the integration of the characteristic equation method, considering external parameters such as temperatures and flow rates of hot, cold, and chilled water circuits as the global heat transfer coefficients. The computational model was built using the F-Chart EES® software, the initial part, to facilitate the resolution of the equations (characteristic equation method). Finally, MATLAB software was used to perform the simulation with the characteristic equation and the transient model. The comparison between the experimental results found in the literature and those obtained by the developed model showed good agreement with relative errors lower than 5% within the values found in the measurement uncertainties. A sensitivity analysis was performed to verify the dynamic behavior of the absorption chiller, considering the activation, cooling, and thermal load temperatures and the products of the global heat transfer coefficients, checking the temperature, pressure, profiles, heat fluxes, and the COP of the chiller. The results showed a coherent behavior when the system was disturbed by varying activation, cooling, and chilled water temperature.

Entropy generation analysis in a gasket plate heat exchanger using non-spherical shape of alumina boehmite nanoparticles

Abstract The analysis deals with the thermo-hydraulic performance of a Gasket Plate Heat Exchanger used for cooling vegetable oils with a water-ethylene glycol 50% and volume fractions of non-spherical nanoparticles mixture as a refrigerant. The heat exchanger has 75 plates with a chevron angle equal to 30º. The Reynolds number of the refrigerant varies from 80 to 1530. The Reynolds number of the sunflower vegetable oil is fixed and equal to 30. The non-spherical nanoparticles used for analysis are platelet, cylindrical and brick types. Graphical results are presented for global heat transfer coefficient, heat capacity ratio, heat transfer rate, outlet temperatures, thermal and viscous entropy generation rate, and Bejan thermodynamic number. The results obtained allow us to conclude that it is possible to work with low relative flow rates using non-spherical nanoparticles, emphasizing platelet nanoparticles. The entropy generations analysis shows that very high flow rates of the refrigerant dissipate much of the energy in viscous form and do not contribute to oil cooling, with a consequent increase in the heat exchanger operating costs.

Heat exchanger design considering variable overall heat transfer coefficient: An artificial neural network approach

AbstractThis study presents an artificial neural network approach in combination with numerical methods to calculate the heat transfer area assuming a nonlinear variation of the global heat transfer coefficient as a consequence of the thermophysical properties of the fluids, the geometry of the surfaces, and other factors. The development of the article is presented in two applications. The first application takes up the database described by Allan P. Colburn, four possibilities are proposed using functions from the field of artificial neural networks to create several approaches. The second application is presented to verify the goodness of the proposed methodology, the artificial neural network model is applied in an experimental data set of double‐pipe vertical heat exchangers, the comparison between the calculated and experimental heat transfer area shows a relative percentage error smaller than 2.8%. The results in the applications are evidence of the competitiveness of the artificial neural network for the prediction of the heat transfer area considering a variable overall heat transfer coefficient.