Determination Of Heat Transfer Coefficient Research Articles

Heat transfer and friction factor of composite TiO2–SiO2 nanofluids in water-ethylene glycol (60:40) mixture

The need for high performance of heat transfer has been evaluated by finding different ways to enhance heat transfer rate in fluid. One of the methods is the combination of two or more nanoparticles and it is known as hybrid/composite nanofluids which can give better performance of heat transfer. Thus, the present study focused on combination of Titanium oxide (TiO2) and Silicon oxide (SiO2) nanoparticles dispersed in 60:40 volume ratio of water and ethylene glycol mixture as the base fluid. The TiO2-SiO2 hybrid nanofluids are prepared using two-step method for different concentration of 2.0%, 2.5% and 3.0%. The experimental determination of heat transfer coefficients are conducted in the Reynolds numbers range from 2000 to 10000 at a bulk temperature of 30°C. The experiments are undertaken for constant heat flux in a circular tube. The Nusselt number of composite TiO2- SiO2 nanofluids is observed to be higher than the base fluid. The finding on heat transfer coefficient shows that 3.0% volume concentration is the highest enhancement with 45.9% compared with base fluid. While at concentration 2.0% and 2.5%, the enhancement recorded were 29.4% and 33.2%, respectively. The friction factor of nanofluids shows a decreased with the increasing of Reynolds numbers. However, the friction factor slightly increased with the increased of concentration.

Boiling of multicomponent working fluids used in refrigeration and cryogenic systems

Working fluids based on mixtures are widely used in cryogenic and refrigeration engineering. One of the main elements of low-temperature units is a recuperative heat exchanger where the return flow cools the direct (cold regeneration is carrying out) resulting in continuous boiling and condensation of the multicomponent working fluid in the channels. The temperature difference between the inlet and outlet of the heat exchanger can be more than 100K, which leads to a strong change in thermophysical properties along its length. In addition, the fraction of the liquid and vapor phases in the flow varies very much, which affects the observed flow regimes in the heat exchanger channels. At the moment there are not so many experimental data and analytical correlations that would allow to estimate the heat transfer coefficient during the flow of a two-phase mixture flow at low temperatures. The work is devoted to the study of the boiling process of multicomponent working fluids used in refrigeration and cryogenic engineering. The description of the method of determination of heat transfer coefficient during boiling of mixtures in horizontal heated channel is given as well as the design of the experimental stand allowing to make such measurements. This stand is designed on the basis of a refrigeration unit operating on the Joule-Thomson throttle cycle and makes it possible to measure the heat transfer coefficient with a good accuracy. Also, the calculated values of the heat transfer coefficient, obtained with the use of various correlations, are compared with the existing experimental data. Knowing of the heat transfer coefficient will be very useful in the design of heat exchangers for low-temperature units operating on a mixture refrigerant.

Measurement of Transient Fluid Temperature in a Pipeline

ABSTRACTThe aim of this paper is to develop a method of determining the heat transfer coefficients on the inner surface of the pipeline and outer surface of the thermometer used to measure the temperature of a fluid flowing under high pressure. The method is based on the solutions to the inverse heat conduction problems for the thermometer and the pipeline wall. The heat transfer coefficients are determined based on the measurement of the temperature of a cylindrical metal thermometer and the temperature of the wall of a cylindrical pipeline. The temperature sensor is located in the pipeline wall close to the inner surface. The correlations for the Nusselt numbers used to determine heat transfer coefficients on the outer surface of the thermometer and the inner surface of the pipeline contain unknown coefficients which are found using the least squares method. The unknown coefficients are selected so that the sum of the squares of differences between the fluid temperature determined based on the measurement of the temperature of the pipeline wall and the fluid temperature obtained from measurements inside the thermometer, calculated for several dozen set time points, is as small as possible.

Development of a Mini-Freeze Dryer for Material-Sparing Laboratory Processing with Representative Product Temperature History

The goal of the work described in this publication was to evaluate a new, small, material-sparing freeze dryer, denoted as the "mini-freeze dryer or mini-FD", capable of reproducing the product temperature history of larger freeze dryers, thereby facilitating scale-up. The mini-FD wall temperatures can be controlled to mimic loading procedures and dryer process characteristics of larger dryers. The mini-FD is equipped with a tunable diode laser absorption spectroscopy (TDLAS) water vapor mass flow monitor and with other advanced process analytical technology (PAT) sensors. Drying experiments were performed to demonstrate scalability to larger freeze dryers, including the determination of vial heat transfer coefficients, K v . Product temperature histories during K v runs were evaluated and compared with those obtained with a commercial laboratory-scale freeze dryer (LyoStar II) for sucrose and mannitol product formulations. When the mini-FD wall temperature was set at the LyoStar II band temperature (-20°C) to mimic lab dryer edge vials, edge vial drying in the mini-FD possessed an average K v within 5% of those obtained during drying in the LyoStar II. When the wall temperature of the mini-FD was set equal to the central vial product temperature, edge vials behaved as center vials, possessing a K v value within 5% of those measured in the LyoStar II. During both K v runs and complete product freeze drying runs, the temperature-time profiles for the average edge vials and central vial in the mini-FD agreed well with the average edge and average central vials of the LyoStar II.

Determination of heat transfer coefficient between AZ91 magnesium alloy casting and no-bake mold

Determination of heat transfer coefficient between AZ91 magnesium alloy casting and no-bake mold

A Study About Performance Evaluation Criteria of Tube Banks With Various Shapes and Arrangements Using Numerical Simulation

Tube bank heat exchangers are designed to efficiently transfer heat between two fluids. Shapes and arrangements of tubes in heat exchangers have significant effects in heat transfer and pressure drop of fluid. In this study, the three-dimensional (3D) numerical investigation is performed to determine heat transfer coefficients, friction factor, and performance evaluation criteria (PEC) of cam-shaped tube banks in aerodynamic and inverse aerodynamic directions in the cross flow air and compared with those of elliptical tube banks in heat exchanger. The arrangements of tubes are aligned and staggered with longitudinal pitch of 44.88 mm and transverse pitch of 28.05 mm. Reynolds number in the range of 11,500–18,500 was used, and the tube surface temperature was fixed and considered 352 K. Results indicate the superior heat transfer of elliptical tube bank over the cam-shaped tube banks in inverse aerodynamic and aerodynamic directions in both arrangements. Moreover, the PEC of the cam-shaped tube banks with inverse aerodynamic and aerodynamic directions and elliptical tube bank in aligned arrangement are approximately 1.4, 1.1, and 1.6, respectively. The obtained results for staggered arrangements are also 1.5, 1.3, and 1.8, respectively.

New design equations for turbulent forced convection heat transfer and pressure loss in pillow-plate channels

Pillow-plate heat exchangers (PPHE) represent a stack of pillow plates characterized by a wavy surface and fully welded construction. The benefits of PPHE over conventional heat exchangers have made them increasingly attractive for numerous applications in the process industry. However, the lack of verified design methods and reference applications for PPHE hinders their widespread application. This work aims at overcoming this “bottleneck” by providing new equations for the thermo-hydraulic design of pillow plates.In particular, design methods for pressure loss and heat transfer are proposed for turbulent forced convection in pillow-plate channels over a wide range of Reynolds numbers (1000≤Re≤8000), Prandtl numbers (1≤Pr≤150) and characteristic geometry parameters (e.g., welding spot arrangement, inflation height). For the determination of heat transfer coefficients, two different approaches are presented. The first approach is based on the method typically suggested in literature, whereby the well-known Dittus-Boelter type power law function (cf. [1]) for the Nusselt number is applied and fitted to numerical data largely obtained in Ref. [2]. The second approach is based on the characteristic flow pattern in pillow plates suggested in Ref. [2]. In this pattern, the initial complex flow is broken down into two simpler flows, which can then be modeled separately. For both methods, the relative deviations between the numerical data obtained in Ref. [2] and in this study are less than 15%.

Determination of the interfacial heat transfer coefficient at the metal-sand mold interface in low pressure sand casting

The interfacial heat transfer coefficient (IHTC) at the metal-sand mold interface was determined by applying a nonlinear inverse estimation method during the low pressure sand casting (LPSC) process. Experiments were conducted using plate shape castings with different thicknesses for both ZL205A and ZL114A aluminum alloys. A specific temperature sensor unit was developed to measure accurately the temperature evolution inside the sand mold, based on which the IHTC was estimated. Results showed that the time-dependent IHTC during the LPSC mainly comprised two stages, i.e., rapid increasing stage and maintain stage, and the second stage could be further divided into two parts based on the plate thickness. Besides, the IHTC was found to be also highly dependent on the casting material, especially the maximum value. Further analysis also revealed that the estimation accuracy of the inverse method was increased by 50% if the temperature dependence of the thermo-physical parameters was considered, rather than normally constant treatment or simple linear interpolation versus temperature.

Supercritical water heat transfer coefficient prediction analysis based on BP neural network

14 groups of experimental data have been collected for determining the heat transfer coefficient within the scope of supercritical water pressure. On the basis of experimental data, the BP neural network predictive model has been built for the determination of supercritical water heat transfer coefficient. Using the BP neural network prediction model, a study has been conducted to determine effect of changing certain parameters such as heat flux, mass flux, pipe diameter and pressure on the heat transfer coefficient of supercritical water. The prediction results show that the mean error, standard deviation and the root mean square error are 0.179032187%, 4.128897482% and 0.179032187% respectively, the regression coefficient R is 0.97165 and the maximum error is 16.06332%. The trained BP neural network prediction model can applied for better prediction and understanding of the heat transfer coefficient of supercritical water. The prediction range is as follow: specific enthalpy is 451.30–3135.87kJ/kg, mass flux is 400–3000kg/m2s, heat flux is 200–2960kW/m2, pressure is 22.6–31MPa and diameter is 0.7–38.1mm.

Determination of the interfacial heat transfer coefficient between forced air and sand at Reynold’s numbers relevant to smouldering combustion

Heat transfer between flowing air and a fixed sand bed at low Reynold’s number (i.e., Re<30) is central to numerous natural and applied processes, including smouldering combustion. The most widely used correlation for the heat transfer coefficient (hsg) predicts Nusselt numbers so high, it effectively presumes local thermal equilibrium for these systems; an assumption that has never been tested. In this work, twelve column experiments combined with numerical modelling quantify hsg across a range of relevant sand grain sizes (0.125<dp<2.000mm) and air flow rates (0.5<Re<31). All of the sand properties were determined independently, with only hsg determined via inverse modelling. A new empirical correlation for hsg is obtained, Nu=0.001 (Re1.97Pr1/3), which is then validated against two additional experiments. A newly developed criterion for assuming local thermal equilibrium is shown to be violated in all of these convection-dominated experiments and the extent of non-equilibrium between sand and air is quantified. The centerline temperatures are demonstrated to be sensitive not only to hsg but also to a global heat loss coefficient quantified from the experiments in a novel manner. Overall, the new hsg correlation is demonstrated to be reliable for predicting the interphase heat transfer in these systems and its application is expected to be valuable for a wide range of processes including smouldering.

Determination of interfacial heat transfer coefficient and analysis of influencing factors in warm forming the third-generation automotive medium-Mn steel

The third-generation automotive medium-Mn steel (TAMM steel) was newly developed. The medium-Mn steel holds the advantages of fine and even microstructure, low austenizing temperature and wide quenching rate range, and thereby it is possible to contribute to excellent properties, high efficiency and low cost in warm/hot forming. However, research on the forming of this TAMM steel has just started. It is necessary to understand the relation between forming process and heat transfer, and further apply the heat transfer characteristics to numerical simulation. Therefore, this paper explored the interfacial heat transfer rules during warm forming the TAMM steel and determined the interfacial heat transfer coefficient (IHTC) using a self-designed experimental platform and Beck's inverse estimation method. The results indicate that the IHTC is not constant but varies nonlinearly during quenching. Moreover, the influences of several factors, including stamping pressure, stamping temperature and surface coating, on the IHTC were investigated. The IHTC increases with the increase of stamping pressure and stamping temperature. However, it decreases greatly with the increase of coating thickness. It is also found that the microstructure and mechanical properties are not sensitive to the coating and quenching rate, which is beneficial to the achievement of even mechanical properties of the stamped TAMM part, and helpful to the flexible design of quenching method and coating application in actual warm stamping process.

A Markov Chain Monte Carlo-Metropolis Hastings Approach for the Simultaneous Estimation of Heat Generation and Heat Transfer Coefficient from a Teflon Cylinder

ABSTRACTThis paper reports the use of Markov Chain Monte Carlo (MCMC) and Metropolis Hastings (MH) approach, to solve an inverse heat transfer problem. Three-dimensional, steady state, conjugate heat transfer from a Teflon cylinder of dimensions 100 mm diameter and 100 mm length with uniform volumetric internal heat generation is considered. The goal is to estimate volumetric heat generation and heat transfer coefficient, given the temperature data at certain fixed location on the surface of the cylinder. The internal volumetric heat generation is specified as input and the temperature and heat transfer coefficient values are obtained by a numerical solution to the governing equation. The temperature values also depend on heat transfer coefficient which is obtained by solving Navier–Stokes equation to obtain flow information. In order to reduce the computational cost, a neural network is trained from the computational fluid dynamics simulations. This is posed as an inverse problem wherein volumetric heat generation and heat transfer coefficient are unknown but the temperature data is known by conducting experiments. The novelty of the paper is the simultaneous determination of volumetric heat generation and heat transfer coefficient for the experimentally measured steady-state temperatures from a Teflon cylinder using MCMC-MH as an inverse model in a Bayesian framework and finally, the estimates are reported in terms of mean, maximum a posteriori, and the standard deviation which is the uncertainty associated with the estimated parameters.

Real-Time Determination of Convective Heat Transfer Coefficient Via Thermoelectric Modules

In this paper, the determination of convective heat transfer coefficient under actual convection processes is proposed by using thermoelectric modules. The thermoelectric modules are positioned where cooling/heating processes take place. Based on the Seebeck effect and energy balance, voltage signals are mathematically related to the convective heat transfer coefficient in real time. In experiments, convective heat transfer coefficients of airflow in a wind tunnel are determined under heating/cooling processes at various wind speeds. The relative mean difference of the convective heat transfer coefficients between the proposed methodology and empirical formula is 2.31%. For real-time implementation, convective heat transfer coefficients of a copper plate, which is exposed to outdoor conditions during a whole day, are determined to predict copper plate temperatures from a governing equation. The performance of temperature prediction is confirmed by a coefficient of determination R2 of 0.9992. Analytical and experimental results show the effectiveness of the proposed thermoelectric modules in determining the convective heat transfer coefficient for air under actual cooling/heating conditions, in time.

Analytical modeling of the thermal aspects of metalworking fluids in the milling process

Heat generated in the cutting zone during metal cutting has an undeniable controlling influence on the dimensional accuracy of the workpiece and tool life. The heat dissipation process in the cutting zone is critical to the type of metalworking fluid (MWF) used and the way it is delivered to the cutting zone. Although extended studies have analyzed the thermal aspect of the machining process, few, if any, have incorporated the influence of the delivery method (application) of the MWF on the convective heat transfer process. In this paper, a comprehensive approach analysis was taken to assess the heat generation in the cutting process. Analytical models were developed to estimate the amount of heat generated in the shear zone and the heat partition in the shear plane during the milling process. In addition, the heat transfer coefficients of metalworking fluids during flooding, minimum quantity lubrication (MQL), and through-spindle cooling (TSC) strategies were evaluated. Regarding MQL, the determination of heat transfer coefficients of metalworking fluids involves the use of a homogeneous single-phase flow assumption of the two fluids (air and lubricant). Next, the average temperature change in the workpiece was estimated by considering the conductive and convective heat transfer coefficients of the workpiece and fluids. Finally, the analytical models of the temperature change in the workpiece were experimentally validated against workpiece-embedded thermocouple measurements. It was found that the results of the models show good agreement with the experimental results.

Analytical and experimental determination of slug flow parameters, pressure drop and heat transfer coefficient in micro-channel condensation

This paper investigates slug flow associated with condensation along a module containing 10 of 29.9-cm long micro-channels having 1×1-mm2 cross section. Using FC-72 as condensing fluid, which is cooled by water counterflow, experiments are performed to measure pressure drop as well as detailed temperature variations along the condensation module. Using high speed video, the flow is shown to consist of a series of unit cells, each comprised of a liquid slug and an elongated bubble. New interfacial instability theory is developed to describe the transition from annular flow to slug flow and obtain analytical expressions for bubble and slug lengths for the most upstream unit cell. Also presented is a new theoretical model for slug flow that is used to determine axial variations of bubble, slug and unit cell lengths. These lengths are used to evaluate axial variations of the local heat transfer coefficient, which are accurately predicted using the new model. Additionally, pressure drop data show good agreement with predictions of a recent universal correlation approach for condensation in small channels.

Determination of the interfacial heat transfer coefficient for a hot aluminium stamping process

The interfacial heat transfer coefficient (IHTC) is an important thermophysical parameter in hot stamping processes and must be identified not only to retain the full mechanical strength of formed components, but also to optimise the production rate. In this work, a novel experimental facility was developed and applied to measure the temperature evolutions of the specimens and tools in stamping processes. Simulated temperature evolutions obtained using the FE software PAM-STAMP were then fit to this data. The IHTC values between AA7075 and three different tool materials were characterized at different contact pressures under both dry and lubricated conditions. In addition, a mechanism based IHTC model was developed and validated as a function of contact pressure, tool material and lubricant thickness to predict the IHTC values under different conditions.

Determination of interface heat transfer coefficient between aluminum casting and graphite mold

To produce castings of titanium, nickel, copper, aluminum, and zinc alloys, graphite molds can be used, which makes it possible to provide a high cooling rate. No die coating and lubricant are required in this case. Computer simulation of casting into graphite molds requires knowledge of the thermal properties of the poured alloy and graphite. In addition, in order to attain adequate simulation results, a series of boundary conditions such as heat transfer coefficients should be determined. The most important ones are the interface heat transfer coefficient between the casting and the mold, the heat transfer coefficient between the mold parts, and the interface heat transfer coefficient into the environment. In this study, the interface heat transfer coefficient h between the cylindrical aluminum (99.99%) casting and the mold made of block graphite of the GMZ (low ash graphite) grade was determined. The mold was produced by milling using a CNC milling machine. The interface heat transfer coefficient was found by minimizing the error function reflecting the difference between the experimental and simulated temperatures in a mold and in a casting during pouring, solidification, and cooling of the casting. The dependences of the interface heat transfer coefficient between aluminum and graphite on the casting surface temperature and time passed from the beginning of pouring are obtained. It is established that, at temperatures of the metal surface contacting with a mold of 1000, 660, 619, and 190°C, the h is 1100, 4700, 700, and 100 W/(m2 K), respectively; i.e., when cooling the melt from 1000°C (pouring temperature) to 660°C (aluminum melting point), the h rises from 1100 to 4700 W/(m2 K), and after forming the metal solid skin on the mold surface and decreasing its temperature, the h decreases. In our opinion, a decrease in the interface heat transfer coefficient at casting surface temperatures lower than 660°C is associated with the air gap formation between the surfaces of the mold and the casting because of the linear shrinkage of the latter. The heat transfer coefficient between mold parts (graphite–graphite) is constant, being 1000 W/(m2 K). The heat transfer coefficient of graphite into air is 12 W/(m2 K) at a mold surface temperature up to 600°C.

Two-layer ETFE cushions integrated flexible photovoltaics: Prototype development and thermal performance assessment

ETFE (ethylene tetrafluoroethylene) cushion integrated flexible photovoltaics (PV) is an extension of building integrated photovoltaics into membrane structures, which could be near zero-energy, sustainable and environmental-friendly buildings. This paper focused on a two-layer ETFE cushion integrated flexible photovoltaics with experimental study and theoretical analysis. Field experiments on a prototype were carried out to investigate temperature distribution and characteristics. It is found that temperature distribution was the result of solar irradiance, incident angle and surface curvature of ETFE cushion and that solar irradiance had an essential effect on temperature distribution. The theoretical thermal model was developed based on energy balance equation and the corresponding differential equation was solved by the Runge-Kutta method. Maximum temperature difference of 3.3K between experimental and numerical results demonstrated that this thermal model could predict PV temperature appropriately. Furthermore, a modified equation to determine heat transfer coefficients was proposed and average heat transfer coefficients of PV and ETFE foil were 4.89W/(m2K) and 4.39W/(m2K). In general, this study could provide basic values and observations for investigating thermal performance of ETFE cushion integrated flexible photovoltaics.

Experimental determination of the heat transfer coefficient in shell-and-tube condensers using the Wilson plot method

This article deals with the experimental determination of heat transfer coefficients. The calculation of heat transfer coefficients constitutes a crucial issue in design and sizing of heat exchangers. The Wilson plot method and its modifications based on measured experimental data utilization provide an appropriate tool for the analysis of convection heat transfer processes and the determination of convection coefficients in complex cases. A modification of the Wilson plot method for shell-and-tube condensers is proposed. The original Wilson plot method considers a constant value of thermal resistance on the condensation side. The heat transfer coefficient on the cooling side is determined based on the change in thermal resistance for different conditions (fluid velocity and temperature). The modification is based on the validation of the Nusselt theory for calculating the heat transfer coefficient on the condensation side. A change of thermal resistance on the condensation side is expected and the value is part of the calculation. It is possible to improve the determination accuracy of the criterion equation for calculation of the heat transfer coefficient using the proposed modification. The criterion equation proposed by this modification for the tested shell-and-tube condenser achieves good agreement with the experimental results and also with commonly used theoretical methods.

Comparative evaluation of methods for the determination of heat transfer coefficients of liquid and gaseous quenching media

Temperature dependences of heat transfer coefficients of liquid and gaseous quenching media were determined using a gradient probe and prismatic probe of more simple design. The probes of two different designs were tested in the same conditions. Analysis of heat transfer coefficients showed good agreement between the data obtained. The tests were carried out with liquid and gaseous quenching media: water, polymer quenchant, quenching oil and high-pressure nitrogen. Methods of mathematical modeling of steel samples quenching show the adequacy of the results.