Apparent Heat Transfer Coefficient Research Articles

Fabrication and Performance of 300-mm Wafer-Scale Silicon Microchannel Cooler

This paper describes the design, fabrication, and thermal performance of 300 mm diameter wafer-scale Si microchannel coolers that demonstrated a junction temperature rise of less than 18°C when dissipating about 14 kW of power. A thermal test wafer, which included hot-spot regions to mimic high-power compute cores, was attached to the wafer-scale microchannel cooler with Pb-Sn solder. Glass manifold layers were attached to the microchannel wafer using a frit material. The average apparent heat transfer coefficient, h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">app</sub> , was about 104,000 W/(K-m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ).

Condensation of NH3/H2O with mass concentrations of 80%-96%: Experimental study in a plate heat exchanger

High concentration NH3/H2O is suitable for Kalina cycles used for the recovery of low grade heat. Plate heat exchangers (PHEs) are compact and reduce the charge of working fluid. This paper investigates the condensation of NH3/H2O with NH3 mass concentrations of 80%-96%. The vapor and liquid concentrations are close to equilibrium state, which are different from normal absorbers. The apparent heat transfer coefficients (HTCs) and frictional pressure drop are presented, covering the mass fluxes of 32–86 kgm−2s−1, the averaged vapor qualities of 0.08–0.65 and the saturated pressure of 610 to 780 kPa. Larger mass fluxes noticeably increase the apparent HTCs and frictional pressure drop. At the mass concentrations of 96%, 91% and 88%, higher vapor qualities increase the apparent HTCs for large mass fluxes. The apparent HTCs decrease slightly with vapor qualities for 80% mass concentration. The experimental results are compared with those of pure NH3. The flow patterns of high concentration NH3/H2O are considered as full film flow and partial film flow, which are the same as for NH3. The mass transfer resistance deteriorates the heat transfer especially for partial film flow, which happens at small liquid mass fluxes. The mass transfer resistance has negligible influences on frictional pressure drop.

Predicting the Temperature Evolution during Nanomilling of Drug Suspensions via a Semi-Theoretical Lumped-Parameter Model.

Although temperature can significantly affect the stability and degradation of drug nanosuspensions, temperature evolution during the production of drug nanoparticles via wet stirred media milling, also known as nanomilling, has not been studied extensively. This study aims to establish both descriptive and predictive capabilities of a semi-theoretical lumped parameter model (LPM) for temperature evolution. In the experiments, the mill was operated at various stirrer speeds, bead loadings, and bead sizes, while the temperature evolution at the mill outlet was recorded. The LPM was formulated and fitted to the experimental temperature profiles in the training runs, and its parameters, i.e., the apparent heat generation rate Qgen and the apparent overall heat transfer coefficient times surface area UA, were estimated. For the test runs, these parameters were predicted as a function of the process parameters via a power law (PL) model and machine learning (ML) model. The LPM augmented with the PL and ML models was used to predict the temperature evolution in the test runs. The LPM predictions were also compared with those of an enthalpy balance model (EBM) developed recently. The LPM had a fitting capability with a root-mean-squared error (RMSE) lower than 0.9 °C, and a prediction capability, when augmented with the PL and ML models, with an RMSE lower than 4.1 and 2.1 °C, respectively. Overall, the LPM augmented with the PL model had both good descriptive and predictive capability, whereas the one with the ML model had a comparable predictive capability. Despite being simple, with two parameters and obviating the need for sophisticated numerical techniques for its solution, the semi-theoretical LPM generally predicts the temperature evolution similarly or slightly better than the EBM. Hence, this study has provided a validated, simple model for pharmaceutical engineers to simulate the temperature evolution during the nanomilling process, which will help to set proper process controls for thermally labile drugs.

Radial heat transport in a fixed-bed reactor made of metallic foam pellets: Experiment and particle-resolved computational fluid dynamics

Open-cell metallic foams in the shape of pelletized catalysts are regarded as potential catalyst substrates for the application in fixed-bed reactors. This work reports an experimental study and a detailed Computational Fluid Dynamics (CFD) model to investigate the heat transfer performance of a fixed-bed reactor made of open-cell metallic foam pellets. The steady-state heat transfer behavior of a hot gas flowing through such a packed bed under wall-cooled conditions was examined. The proposed CFD model is in line with the particle-resolved CFD (PRCFD) approach that accounts explicitly for the random packing structure, so that the transport processes in the interstitial regions are fully resolved. The momentum and energy transports inside the highly porous foam particles are considered by closure equations based on the porous-media model. The Rigid Body Dynamics (RBD) method is adopted to create the packing geometry. The axial temperature and the pressure drop obtained by the CFD simulations show good agreement with experimental data. To evaluate the thermal performance, effective heat transport parameters in a packed bed such as the effective radial thermal conductivity and the apparent wall-fluid heat transfer coefficient are determined, with the aid of a 2D pseudo-homogenous plug-flow reactor model. The correlations for heat transport characteristics as a function of particle Reynolds number for the metallic foam packed bed are also presented.

Examination of the Heat Transfer Model in Gas–Liquid–Solid Three Phases in Presence of Gas Hydrate Particle

Hydrate-based gas separation is often investigated using batch or semi-batch operations. To increase the throughput of the gas mixture without increasing the apparatus volume, it is preferable to perform a continuous operation of hydrate-based gas separation. Therefore, we proposed a flow-type apparatus for performing continuous formation with passing gaseous mixture and subsequently decomposition with passing gas hydrate particles. Characteristics of multiple fluid and heat and mass transfer of hydrate slurry are essential for the efficient operation of the apparatus. In this study, we focused on heat transfer characteristics in the presence of bubbles in water and surfactant solution. First, an apparent overall heat transfer coefficient under pressure during steady operation of the apparatus was calculated on a simple assumption. Next, to control the hydrate amount and position of hydrate-decomposition and hydrate-formation in the apparatus, we focused on the temperature profile of the inside fluid. A heat transfer model using heat balance of defining heat of hydrate-formation and heat transfer of agitation of fluid was made for hydrate-based gas separation apparatus. To evaluate the validity of the heat transfer model, a calculation value is compared with the experimental value.

Simple models for predicting water loss of bread during baking

AbstractThe baking process has received quite attention during the last three decades, due to its economic and industrial relevance, and since it is associated with staple foods like bread. Because of the composition and structure of dough, a series of complex, simultaneous and coupled phenomena occur in the product inside the oven, which make very difficult the design, control and optimization of the process. For these tasks, the use of mathematical models appears as an adequate solution, although they still represent a complex approach for industrial purposes and their use is limited. In this paper, we focus on prediction of water loss, which is relevant for different aspects of the process, for example, texture and sensory properties, economic impact, energy demand. Two simple models are developed based on knowledge about transport phenomena of bread baking. Both methods involve the estimation of a mass transfer coefficient, which can be related to operating conditions. Good prediction performance is found in both cases, in a common range of baking conditions. Finally, the use of water loss as a design and control variable of the process is discussed, and we propose the construction of a baking diagram with moisture loss as independent variable.Practical applicationsOverall, there is a lack of simple and accurate prediction methods for baking, in contrast with other traditional food process operations, since the complexity of the process makes very difficult the task of developing user‐friendly models. This work proposes two simple models to predict water loss variation during bread baking, based on knowledge about transport phenomena of the process. Implementation of the proposed methods needs simple adjustment of parameters, which can be related to operating conditions (oven temperature and apparent heat transfer coefficient). Besides, the construction of a baking diagram is discussed.

Flow behaviors and heat transfer characteristics of liquid film during the pyrolysis process of molten plastics using OpenFOAM

Falling film pyrolysis can improve the quality of pyrolysis products of waste plastics. The pyrolysis process of molten plastics is a complex process and the interphase force has an important influence on the flow and heat transfer process of the liquid film. Based on OpenFOAM® and considered the interphase drag force, the flow behaviors and heat transfer characteristics of the liquid film during the falling film pyrolysis of molten plastics were numerically simulated by VOF (Volume of Fluid) method. The results show that the surface of the liquid film is a wavy structure and the liquid film presents a comprehensive flow pattern of film, rivulet and droplet. With the increase of pyrolysis temperature, the length of liquid film and the average apparent heat transfer coefficient decrease, and the average thickness increases slightly. With the increase of the liquid film inlet velocity, the liquid film thickness and the average apparent heat transfer coefficient increase.

Heat transfer characteristics of molten plastics in a vertical falling film reactor

Abstract The heat transfer efficiency during the pyrolysis process is a key factor to be considered in the design of pyrolysis reactors. In this study, the average apparent heat transfer characteristics of molten plastic pyrolysis in a vertical falling film reactor were explored by experiments and numerical simulation and the apparent heat transfer coefficients were determined. In addition, the temperature distribution and the thickness of the liquid film in the reactor were predicted and the influences of pyrolysis temperatures on the average apparent heat transfer coefficients were discussed.

Heat transfer trait simulation of H finned tube in ventilation methane oxidation steam generator for hydrogen production

With the steam obtained by the energy released from ventilation air methane oxidation, the hydrogen production through gasification method is considered more commercial. In order to constrain the steam parameter fluctuation, the shunt honeycomb ceramics are adopted to fill between the heat exchange tubes. However, transient heat transfer characteristics of this kind of heat exchanger have not been fully studied. This paper carried out a numerical simulation study on the transient heat transfer characteristics of H finned tube under periodic reverse-flow conditions. Results show that the existence of shunt honeycomb ceramics enhances the effect of radiation. Gas flow direction reversing destructs the original boundary layer, achieving a sudden rise of the convective heat flux in the new upstream. It takes about 12s to form a new relative stable boundary layer. The apparent heat transfer coefficient achieves a maximum of 108.77 W m−2 K−1 and an average of 99.1 W m−2 K−1.

Improving thermal management of electronic apparatus with paraffin (PA)/expanded graphite (EG)/graphene (GN) composite material

This study introduced a novel PA-EG-GN composite phase change material (PCM) with better thermos-physical properties to help dissipate heat in electronic apparatus. Both the X-ray diffractometer and Fourier transformation infrared spectra patterns show that the composite PCM is the pure physical combination of no chemical interactions. Raman spectroscopy results suggest that the structural symmetry of the GN decreases with vibration increases. Results from Scanning Electron Microscopy show that GN can improve the compatibility with the mixture of PA and EG. Differential scanning calorimetry curves indicate that the composite PA-EG-GN has a lower latent heat than that of pure PA and PA-EG. The weight-loss ratios of the PA-EG and PA-EG-GN are roughly equivalent to the mass ratio of PA in the composite PCMs. The thermal conductivity of the PA-EG-GN is evidently higher than PA-EG and shows a strong linear relationship with the compress density. To verify these conclusions, an experiment was conducted to compare the thermal management capabilities of the PA-EG composites and the PA-EG-GN composites with several simulative chips. Both the surface peak temperature and the apparent heat transfer coefficient of chips were measured. The final results confirmed that the PA-EG-GN has a better performance than regular PA-EG composites.

Case studies of heat conduction in rotary drums with L-shaped lifters via DEM

Rotary drums are widely used in numerous processes in industry to handle granular materials. In present work, heat transfer processes in drums with L-shaped lifters have been investigated by coupling the discrete element method (DEM) with heat transfer model. Effects of both operational and structural parameters have been analyzed. It is found that increasing rotational speed could improve heat transfer to a certain extent, however, just in relatively low speed stage. When lifter number increases, the heat transfer speed slightly decreases. An increasing lifter height could promote heat transfer first and then reduces it, but the amplitude of variation keeps small. The heat transfer rate descends with increasing lifter width. The heat transfer mechanisms have also been discussed by comparing mixing rates, total contact areas for thermal conduction, time constants (TC) indicating apparent heat transfer rate and effective heat transfer coefficients(HTC). It is concluded that dynamic conduction due to particle flow is dominated in all cases. The L-shaped lifers are turned out not a good choice when heat conduction between particles is prominent.

Investigation on temperature fields in industrial chilling of pork legs for PDO Parma ham production and coupled temperature/microbial growth simulation.

In the present work, temperature distribution within an industrial blast chiller with pork leg for Protected Designation of Origin Parma ham production was studied and the slowest cooling zone and the fastest cooling zone were recognized. Moreover, apparent heat transfer coefficients in both positions were calculated and resulted equal to 27.1 and 15.6Wm-2°C-1, respectively. A finite element method model by distinguishing the three main components (skin, lean meat and bone) of the leg for the unsteady heat transfer during cooling was developed and validated against industrial chiller data. Good agreement between experimental and simulated data was obtained with RMSE value for thermal centre equal to 0.81°C. Furthermore, in order to introduce CCP limit in the HACCP plan, a microbial growth model of the most important pathogens in meat was developed starting from the heat transfer model results. Temperature as well as pathogen growth were estimated in the case of different plant breakdowns. Defined CCP limit was represented by reaching 11 °C at 5 mm of depth within 2 h from the beginning of the cooling process, moreover a different cooling program was simulated and established as the alternative one.

The Effects of Interfacial Heat Transfer Coefficient on the Microstructure of High-Pressure Die-Cast Magnesium Alloy AM60B

This paper describes the details of a quantitative experimental and numerical study on the influence of solidification conditions, including the apparent interfacial heat transfer coefficient (IHTC) between the die and solidifying metal, on the resulting local microstructure. Multiple runs of the commercial casting simulation package, ProCASTTM, are used to model the mold filling and solidification events employing a range of IHTC values. The simulation results are used to estimate the centreline cooling curve at various locations through the casting. The centreline cooling curve, together with the die temperature and the thermodynamic properties of the alloy are then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting. Finally, the local cooling rate is used to calculate the resulting grain size and skin thickness via previously established relationships. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent IHTC, which, in this study, was approximately 12000 W/m2·K. Additional useful observations from the numerical study suggest that the IHTC has a significant influence on the skin thickness and grain size in both the skin and core regions of the casting, while the effect of die temperature is limited to influencing the skin grain size only.

Grain Size Distribution and Interfacial Heat Transfer Coefficient during Solidification of Magnesium Alloys Using High Pressure Die Casting Process

The objective of this study is to predict grain size and heat transfer coefficient at the metal–die interface during high pressure die casting process and solidification of the magnesium alloy AM60. Multiple runs of the commercial casting simulation package, ProCAST™, were used to model the mold filling and solidification events employing a range of interfacial heat transfer coefficient values. The simulation results were used to estimate the centerline cooling curve at various locations through the casting. The centerline cooling curves, together with the die temperature and the thermodynamic properties of the alloy, were then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting. Finally, the local cooling rate was used to calculate the resulting grain size via previously established relationships. The effects of die temperature, filling time and heat transfer coefficient on the grain structure in skin region and core region were quantitatively characterized. It was observed that the grain size of skin region strongly depends on above three factors whereas the grain size of core region shows dependence on the interfacial heat transfer coefficient and thickness of the samples. The grain size distribution from surface to center was estimated from the relationship between grain size and the predicted cooling rate. The prediction of grain size matches well with experimental results. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent interfacial heat transfer coefficient for different locations.

Condensation of ammonia and high-temperature-glide ammonia/water zeotropic mixtures in minichannels – Part I: Measurements

In this two-part study, a comprehensive investigation of the condensation of pure and zeotropic mixtures of ammonia and ammonia/water is conducted and improved models for condensation are developed. In Part I, condensation heat transfer experiments were conducted on ammonia and ammonia/water mixtures. Data for pure ammonia were obtained for varying tube diameters (0.98<D<2.16mm), mass fluxes (75<G<225kgm−2s−1), and saturation conditions (30<Tsat<60°C). Experiments on zeotropic ammonia/water mixtures were conducted for multiple tube diameters (0.98<D<2.16mm), mass fluxes (50<G<200kgm−2s−1), and bulk ammonia mass fractions (xbulk=0.8, 0.9 and >0.96). For zeotropic mixtures, the apparent heat transfer coefficient showed a different trend from the pure ammonia heat transfer coefficient, with strong degradation observed in quality ranges with large temperature glides. At low quality ranges (i.e., low temperature glides and smaller mixture effects), the pure ammonia heat transfer coefficient and zeotropic mixture apparent heat transfer coefficient were in good agreement. In Part II of this study, a new model for predicting condensation heat transfer for pure ammonia in mini- and microchannels is introduced.

Modeling of In-Tube Condensation of Zeotropic Mixtures

Studies in the literature have shown that zeotropic mixture condensation rates are lower than those predicted using a pure-fluid approach. This has been attributed to the decrease in fluid temperature that occurs with zeotropic mixtures and to the development of concentration gradients in the vapor-phase that limit the condensation heat transfer. The decrease in the apparent heat transfer coefficient is not consistent across mass fluxes, tube diameters, fluid combinations, saturation pressures, and concentrations. Several modeling techniques exist, which allow engineers to model the decrease in heat transfer rates. This study provides guidelines on when the mass transfer effects can be neglected and when it is appropriate to apply established models in the literature. A condensation database containing fluid combinations of pairs of hydrocarbons, ammonia and water, and synthetic refrigerants across large changes in operating conditions, tube diameters, and concentrations is used to validate the approach. The proposed framework predicts that the Bell and Ghaly (1973, “An Approximate Generalized Design Method for Multicomponent/Partial Condensers,” AIChE Symp. Ser., 69, pp. 72–79) approach is valid for mid- and high-reduced pressures, i.e., above 0.40, while explicitly accounting for mass transfer is necessary at lower reduced pressures, i.e., below 0.40, where the influence of the temperature glide in the Bell and Ghaly method is weighted too strongly.

Modeling of Heat Transfer Effective Coefficient of Ceramic Porous Media by Self-Consistent Method

Providing a comprehensive model for effective heat transfer coefficient using effective medium approximation and with regard to the homogeneity and isotropic thermal insulation materials is very useful in the design process and also to appropriately assess the porous media at different temperature conditions. This case has been studied and presented by many scientists, including Ohm, Russell, Maxwell and etc. However most of them have neglected the direct effect of the fluid within the cavity. In this study, the possibility of generalized self-consistent model with the ability to accurately estimate the effective coefficient of heat transfer in porous media that the 3 parameters of apparent conduction heat transfer coefficient of air, conduction heat transfer coefficients matrix and porosity are considered analytically was examined for the porous material. Apparent conduction heat transfer coefficient of air would indirectly include the effect of the size and shape of the pores, the temperature gradient, the average film temperature, thermo-physical properties of air trapped in the matrix and the dispersed phase. Finally, effective heat transfer coefficient for porous media ceramic insulators that avoid heat dissipation in heating furnace petrochemical olefin units, were calculate using the software Fluent and the results were compared with experimental results. Good agreement between experimental and theoretical results was obtained with a standard deviation of approximately 1%.

Effect of Different Industrial Pasteurization Conditions on Physicochemical Properties of Egg‐Filled Pasta

AbstractThe aim of this study was to estimate the effect of heat treatment using two different industrial pasteurizers (a nascent steam pasteurizer [NSP] and an inject steam pasteurizer [ISP]) on the moisture content, degree of gelatinization, color and furosine content of egg‐filled pasta samples. Moreover, a comparison of samples treated with microwave pasteurization on the packaged product was performed.The results showed that the two pasteurization treatments resulted in significant differences in the sample properties and that all of the parameters correlated well with the thermal treatment intensity (C0) (r value ranged from 0.989 to 0.999; P &lt; 0.05). The application of microwave heat treatment to packaged samples did not significantly alter these differences.The NSP demonstrated lower “thermal damage” with less furosine formation in the pasta and filling; it also reduced certain positive effects, such as starch gelatinization and the presentation of yellow color in terms of hue angle.This difference might be explained by the different apparent heat transfer coefficients observed for the two pasteurizer machines, with the higher value observed for the ISP (347 W/m2 K versus 202 W/m2 K).Practical ApplicationsPasteurization is the most important step in the industrial fresh pasta process because of its effect on the reduction of the microbial population as well as on the maintenance of the product quality and nutritional characteristics. Thus, the study on the effect of different industrial heat treatment conditions on some pasta properties can benefit the future of food pasta industry.This research focused on the pasta modifications using different industrial pasteurizers (a nascent steam versus an inject steam pasteurizer).Our results confirmed (1) modifications due to heat treatments and (2) correlations between the indices used to quantify the modification and thermal treatment intensity. Furthermore, the utilization of different types of pasteurizers used in industrial fresh pasta production resulted in a significant effect on pasta modifications due to different values of the apparent heat transfer coefficients. Hence, the type of pasteurization equipment used was crucial to determine the final quality of the product.

Mathematical Modelling of Heat Transfer in Mortadella Bologna PGI during Evaporative Pre-Cooling

Abstract Mortadella evaporative pre-cooling process from 70 to 50°C at core was investigated: the thermal diffusivity and the apparent heat transfer coefficient were experimentally estimated. The effects of ventilation and water spraying with different intervals (0, 5, 10 and 15 min) were tested and core and surface temperatures, cooling times and cook values were compared. Water spraying every 5 min combined with ventilation allowed obtaining both lowest cooling time and cook values in the product. On the contrary, continuous spraying (no interval) presented higher cooling times since probably, after a certain time, water formed a continuous film on product surface, which prevented evaporation. Based on the experimental data, a finite differences mathematical model, previously applied to Mortadella cooking process, was developed and validated by means of two cooling procedures. Acceptable approximation and low percentage errors on final core temperature were obtained, confirming the usefulness and reliability of the proposed model.

MATHEMATICAL MODELING OF A LABORATORY METHYLCYCLOHEXANE DEHYDROGENATION REACTOR AND ESTIMATION OF RADIAL THERMAL CONDUCTIVITIES AND WALL HEAT TRANSFER COEFFICIENTS

A laboratory micro-pilot fixed-bed reactor for the dehydrogenation of methylcyclohexane was modeled. The kinetic model developed in our previous work (Usman et al., 2012) over 1.0 wt.% Pt/γ-Al2O3 was used in modeling the reactor. The experimental data over the same catalyst was compared with the proposed model and reasonable estimates of the data were found. Effective radial thermal conductivity and apparent wall heat transfer coefficient were observed to be a function of the composition of the reaction mixture and varied along the length of the reactor.