Film Heat Transfer Research Articles

Interaction analysis of a concentric component evaporator absorber for an absorption heat transformer

This work focuses on the analysis of a concentric component evaporator-absorber for an absorption heat transformer for water distillation by a H2O-LiBr refrigerant-absorbent pair. To understand the interrelationship of evaporation and absorption in this component and to identify the limiting mass and energy phenomena, a numerical model was built. This model describes the falling film heat transfer and the distillation inside of tubes per ring of coil. To make a precise specification of the conditions at which the transport properties are evaluated, the MOSAIC modeling framework was used. The model results were compared with experimental tests at steady state. Also, the effect of an increase in the composition of the absorption mixture was studied. The model was also useful to analyze the effect of the operating pressure in the dual component. The results showed that with an increase in the pressure from 17.68 to 21.33 kPa, the heat load in the evaporator decreases from 1.73 to 1.57 kW, and as result, the distilled water in the absorber decreases from 5.5×10-4 to 4.4×10-4 kg/s. This computational model identifies the effect of wetting efficiency in the evaporator as the limiting factor.

Approximate solutions for the problem of liquid film flow over an unsteady stretching sheet with thermal radiation and magnetic field

The proposed method is based on replacement of the unknown function by a truncated series of the shifted Legendre polynomial expansion. An approximate formula of the integer derivative is introduced. Special attention is given to study the convergence analysis and derive an upper bound of the error for the presented approximate formula. The introduced method converts the proposed equation by means of collocation points to a system of algebraic equations with shifted Legendre coefficients. Thus, after solving this system of equations, the shifted Legendre coefficients are obtained. This efficient numerical method is used to solve the system of ordinary differential equations which describe the thin film flow and heat transfer with the effects of the thermal radiation, magnetic field, and slip velocity.

A numerical investigation on the conjugate heat transfer of thin liquid film of water in closed microcavity

The understanding of the conjugate heat transfer in an evaporating thin-film and an intrinsic meniscus is important for enhancing the thermal performance of phase-change heat transfer devices. In this paper, we numerically investigated the conjugate heat transfer in the thin film and the intrinsic meniscus under their own vapor in closed microcavities with different geometries. The results show that, with decreasing microcavity size, the percentage contribution of the thin liquid film to the overall heat transfer increases obviously. However, in order to optimally utilize the thin-film heat transfer, both the microcavity size and the length-height ratio need to be concerned for their effects on the thin-film contribution to the net heat transfer. With increasing superheats that are smaller than 5 K, the contribution of the thin film to the net heat transfer rate becomes somewhat smaller, although the variation seems to be less sensitive to the superheat than the microcavity size. Meanwhile, when the wettability improves, the contribution of the thin film to the net heat transfer rate increases, but the variation of the thin-film contribution is relatively less sensitive to the wettability than the microcavity size and the superheat.

Heat transfer in a falling laminar liquid film with in-depth radiation absorption

A heat transfer problem involving a steady-state falling liquid film along a vertical wall with an imposed constant heat flux on the liquid film surface and an in-depth thermal radiation absorption in the film layer is analyzed. The liquid film is assumed to be normal to the incident radiative heat flux, and the heat flux is assumed to be parallel and unidirectional. Nondimensionalized governing equations are formulated and solved numerically. Sample calculations are provided. The effects of pertinent dimensionless parameters on the heat transfer process are demonstrated. The calculated results reveal that the in-depth thermal radiation absorption would play a key role in the heat transfer in the liquid film.

Exact harmonic solutions to Guyer–Krumhansl-type equation and application to heat transport in thin films

A system of hyperbolic-type inhomogeneous differential equations (DE) is considered for non-Fourier heat transfer in thin films. Exact harmonic solutions to Guyer–Krumhansl-type heat equation and to the system of inhomogeneous DE are obtained in Cauchy- and Dirichlet-type conditions. The contribution of the ballistic-type heat transport, of the Cattaneo heat waves and of the Fourier heat diffusion is discussed and compared with each other in various conditions. The application of the study to the ballistic heat transport in thin films is performed. Rapid evolution of the ballistic quasi-temperature component in low-dimensional systems is elucidated and compared with slow evolution of its diffusive counterpart. The effect of the ballistic quasi-temperature component on the evolution of the complete quasi-temperature is explored. In this context, the influence of the Knudsen number and of Cauchy- and Dirichlet-type conditions on the evolution of the temperature distribution is explored. The comparative analysis of the obtained solutions is performed.

Experimental investigation of R410A and R32 falling film evaporation on horizontal enhanced tubes

In this study, heat transfer performance on horizontal copper tubes of high pressure refrigerants R32 and R410A was investigated. An integrated fin (condensation enhanced) tube and three boiling enhanced tubes with 3-D different enhancement structures were tested. A plain tube was also tested for comparison. Effects of film flow rate, saturation temperature and heat flux on the falling film heat transfer coefficients were investigated. A tube bundle comprised of 6 enhanced boiling tubes was also tested to find the bundle effect. Experiments were carried out at saturation temperatures of 6, 10 and 16 °C, heat fluxes from 20 to 150 kW·m−2 and film flow rates from 0.01 kg·m−1·s−1 to 0.14 kg·m−1·s−1. It is found that the effect of film Reynolds number on HTCs of enhanced tubes can be separated into two regimes, a quasi-plateau regime and a sharp decrease regime. HTCs increase with heat flux. Increase of saturation temperature has negligible effect on enhanced tubes as well as the tube bundle. The integrated fin tube performs best among the tubes tested. As a whole, R410A is inferior than R32. Tubes positioned below the top row possess the similar variation trend of HTCs with that of the first row but suffer an earlier dryout.

Heat transfer enhancement at increasing water concentration in alcohol in the process of non-stationary film boiling

New experimental data on heat transfer in pool film boiling of subcooled ethanol-water mixtures at spherical surfaces are considered. The water solutions with ethanol mass fraction from 10 to 91% and temperature of liquid 50°C were examined. All the experiments were conducted under atmospheric pressure, using the stainless steel sphere of 39 mm in diameter as a cooled body. The sphere was heated up to 450-750°C, depending on ethanol concentration, and immersed into the experimental vessel with subcooled mixture. As it is expected, boiling heat transfer intensifies with ethanol concentration decrease, and duration of cooling decreases. It means that stable film boiling duration decreases, and earlier transition to intensive heat transfer regime occurs.

Numerical Investigation of the Effect of Inserted Twisted Tape inside Submerged Bundle Tubes on its Thermal Performance

Twisted tape insertion in smooth plain tube is one of types of passive methods that is used to enhance heat transfer. Swirl fluid flow inside tube and related heat transfer characteristics are very complex. ANSYS FLUENT (V 16.1) and ASPEN industrial program are used in analyzing this technique for enhancement heat transfer. A circular plain tube has length L=8534mm and 17 mm inner diameter with twisted tape has twist ratio of y = (H/D) = (150/17) =8.8 along with a plain tube were considered for this study. Eight Reynolds numbers (Re) of 784, 1000, 2000, 3000, 4000, 5000, 6000 and 7000 are used to analyze the response of thermal performance. Crude oil API 28 exit temperature, film heat transfer coefficient, Nusselt number and overall enhancement ratio results are presented for both empty and inserted plain tube with comparison between the two cases. An increase of 0.76 to 2.36 overall enhancement is predicted with twist ratio 8.8 for Reynolds number 784 to 7000 respectively.

CFD-DEM modelling of circulation frequencies and residence times in a prismatic spouted bed

Coupled CFD-DEM simulations of a three-dimensional prismatic spouted bed with liquid injection were performed. Spouting stability is investigated by means of pressure drop fluctuation analysis and high speed camera recording. To determine the influence of flow stability on coating quality, droplets are injected in a post-processing step and deposited on colliding solid parcels. In order to estimate liquid holdup in the bed, heat transfer and subsequent evaporation of the surface liquid film were modelled. The coating quality of different process conditions is compared for a simulation time of 10s and found to be broader distributed in the dense stable spouting operating regime. The analysis of particle circulation frequencies shows slower circulation under stable spouting conditions, resulting in a slower coating progress. This explains the inhomogeneous coating quality in the first 10s for these cases. Nevertheless, the frequencies are less broadly distributed compared to the instable regime, which is advantageous for obtaining a homogeneous layer in real-time coating processes.

Numerical study on condensation heat transfer and pressure drop characteristics of ethane/propane mixture upward flow in a spiral pipe

Spiral wound heat exchanges (SWHE) has been the most widely used in large-scale liquid natural gas (LNG) plants. However, few studies have been focused on the condensation heat transfer and pressure drop for hydrocarbon mixture refrigerant in SWHE tube side. In this paper, the condensation heat transfer and pressure drop characteristics for ethane/propane mixture upward flow in a spiral pipe were numerically investigated. The numerical model was established and verified based on the existing experimental results and new flow pattern observation experiments. It discussed the variation trends of void fraction, frictional pressure drop, heat transfer coefficient and heat and mass transfer resistance with various parameters, such as, vapor quality, mass flux, heat flux, saturation pressure and inclination angle. Comparing with the existing correlations, the results showed that Steiner’s correlation, modified Fuchs’s correlation, Boyko’s correlation could better predict the void fraction, frictional pressure drop, film heat transfer coefficient with mean absolute deviation of 6.08%, 10.67% and 13.06%, respectively. Meanwhile, modified Silver approach was used to modify the mixed effects of ethane/propane mixture on heat transfer. The study provides some constructive instructions to understand the condensation of zeotropic mixtures in the spiral pipe, and is helpful in designing more effective SWHE.

Exact harmonic solution to ballistic type heat propagation in thin films and wires

The system of hyperbolic type differential equations is considered for non–Fourier heat transfer in thin films and wires. Structures of the harmonic solutions for telegrapher equation and for Guyer–Krumhansl equation are identified and compared with each other. The exact harmonic solution is obtained; the influence of the phonon heat transfer is explored. The maximum principle violation is demonstrated. Frequency dependence of the solution for Guyer–Krumhansl equation is shown. The exact analytical solution is derived for the inhomogeneous ballistic–diffusion set of differential equations, governing heat propagation in thin films. The contribution of the ballistic, wave and diffusive heat transfer components as well as the influence of the initial conditions and of the Knudsen number on heat conduction in thin films and wires are investigated. The cases of identical and different from each other Knudsen numbers for ballistic and diffusive components are explored. The examples of exact solutions for the heat transfer in each case are demonstrated.

Polymeric film application for phase change heat transfer

The paper gives a concise review on polymer film heat exchangers (PFHX) with a focus on polyether ether ketone (PEEK) foil as heat transfer element, mechanically supported by a grid structure. In order to promote PFHX applications, heat transfer performance and wetting behavior are studied in detail. Surface modifications to improve wetting are discussed and correlations are presented for critical Reynolds numbers to sustain a stable liquid film. Scaling phenomena related to surface properties and easily adaptable cleaning-in-place (CIP) procedures are further content. The contribution of the foil thickness and material selection on thermal performance is quantified and a correlation for enhanced aqueous film heat transfer for the grid supported PFHX is given. The basic research results and the design criteria enable early stage material selection and conceptual apparatus design.

Studies on the subcooled boiling in a fuel assembly with 5 by 5 rods using an improved wall boiling model

An improved wall boiling model with consideration of thin film heat transfer was employed with the Eulerian two-fluid model to investigate the three-dimensional flow boiling characteristics in a fuel assembly with 5 by 5 rod bundle and a vaned grid. Models were validated by using the experimental data for subcooled boiling, post-dryout heat transfer and critical heat flux. The calculated results agreed well with experimental data. Thermohydraulics in the rod bundle were obtained, including temperature, velocity, phasic volume fraction and pressure, based on which, the effects of mixing vane on the localized thermohydraulics were studied. Mixing vane can significantly reduce the cross section averaged vapor fraction and increase the heat transfer capacity due to the swirl effects on two-phase flow; however, it will increase the localized vapor fraction on the heated surface, which may result in the anticipation of boiling crisis, i.e., reach the critical heat flux. Besides, influences of heat flux profile on the flow and heat transfer characteristics were also obtained by comparing two cases with uniform and cosine heat flux distribution along the axial direction. Finally, the impacts of thin film heat transfer on the wall heat partition were investigated, proving that thin film heat transfer played an essential role in the modeling of wall boiling when the vapor fraction at heated surface exceeding 0.25.

Effect of SiC and ZnO Nanoparticles on UV Absorbance and Heat Transfer of PET Composite Film

Zinc oxide (ZnO) and silicon carbide (SiC) nanoparticles were compounded with polyethylene terephthalate (PET) by using a twin screw extruder, and their effect on the UV absorption and heat transfer in PET/ZnO and PET/SiC composite films was investigated. The presence of ZnO and SiC in the PET matrix was verified by X-ray diffraction. The UV absorbance of both PET/ZnO and PET/SiC composite films increased with increasing particle content. The UV absorbance of PET/SiC was higher than that of PET/ZnO under the same particle content. A thermal imaging camera was used to analyze the heat transfer in PET/ZnO and PET/SiC composite films after heating them to 40 °C. As the content of ZnO and SiC particles increased, the temperature decreased more rapidly. PET/SiC showed faster cooling than PET/ZnO because of the higher thermal conductivity of SiC compared to that of ZnO. Response temperature surfaces for PET/ZnO and PET/SiC were obtained by using an exponential decay function and a second order equation, and the elapsed times to cool to room temperature were estimated.

HEAT AND MASS TRANSFER AND RESISTANCE CHARACTERISTICS OF EVAPORATIVE AIR COOLER WITH HIGH FINNED-TUBE BUNDLES

A theoretical model was suggested and experimental analysis of heat and mass exchange mechanism of a high finned-tube evaporative air cooling device was made. Experiments showed that the heat transfer of the upper four rows of tubes was higher than that of the lower four rows. The upper and lower layers of the proportion of water spray experiments were carried out. In the experimental range, the higher the proportion of the upper water spray, the greater the overall heat transfer. Besides, evaporation and heat transfer of water film were not only related to the air flow and spray water intensity, but also were affected by the relative humidity of imported air, with decrease in the relative humidity of imported air decreased, and with evaporation and heat transfer between air and water film strengthened. According to the experiments, the correlations of evaporation loss and heat transfer were fitted by air flow, spray water intensity, and relative humidity, which showed close agreement with the experimental results to within ± 11% and ± 9%, respectively. The correlation of pressure drop in the high finned-tube bundle was related to air flow rate and the intensity of the spray water, and the fitting error was within ± 8%. This study was instructive for the design of wet air cooler with high finned-tube bundle.

Nanofluid thin film flow and heat transfer over an unsteady stretching elastic sheet by LSM

This study is carried out on the unsteady flow and heat transfer of a nanofluid in a stretching flat plate. Least square method is implemented for solving the governing equations. It also attempts to demonstrate the accuracy of the aforementioned method compared with a numerical one, Runge-Kutta fourth order. Furthermore, the impact of some physical parameters like unsteadiness parameter (S), Prandtl number (Pr) and the nanoparticles volume fraction (ϕ) on the temperature and velocity profiles is scrutinized carefully. Accordingly, the results obtained from this study reveal that the temperature enhances by means of augmenting the nanoparticles volume fraction. At η ∈ {0, 0.5}, the velocity decreases as a result of a rise in nanoparticles volume fraction and at η ∈ {0.5, 1}, an opposite treatment takes place. Moreover, velocity distribution augments by raising the S value, however an inverse trend is observed in temperature values. Moreover, the local skin friction coefficient indicated a notable rise by increasing the S parameter as well as a steady decrease by rising ϕ. Finally, water-Alumina nanofluid demonstrated better heat transfer enhancement compared to other types of nanofluids.

Research on coupled heat transfer of film cooling in LOX/GH<sub>2</sub> thrust chambers

Research on coupled heat transfer of film cooling in LOX/GH<sub>2</sub> thrust chambers

Shadowgraphic Imaging of Fibre-Delivered Pulsed IR Laser-Induced Heat Transfer across Thin Aluminized Polymer Film

Shadowgraphic imaging was employed to investigate the mid-IR laser induced heat transfer through a double layer thin film. The effect of thin metal coat on the polymer film enhanced the transfer of heat and shock waves due to rapid thermal expansion and the explosive evaporation of the thin fluid layer. Sixty two percent of deposited heat expended for water enthalpy and 38% for other factors. A power of 8.8 kW was launched at the surface of aluminium. The thermal coupling of 45% further reduced the input energy to the film and the non-adiabatic heat diffusion (i.e., ) was transmitted instantaneously within the metal with very small loss. The temperature at the surface of the film was determined ≈301 K, well below the aluminium melting point. The Biot number showed that the metal as single layer and the whole film as double layer satisfies the thermally thin film (i.e., ). Considering the Newtons’s law of cooling, the overall film heat transfer coefficient was found 3 k W·m-2·K-1 equivalent of 3.3 × 10-3 W·m2·K-1 thermal resistance. The analysis of images indicated a reducing percentage of heat transfer as a function of delay time based on the comparison of volume ratios. A calculated power of ≈3 kW was transmitted from the rear side of the film sufficient to thermalize the surrounding water layer and form vapor bubble.

Mitigation of biofouling in polymer film heat transfer applications

Mitigation of biofouling in polymer film heat transfer applications

Molecular dynamics study of wettability and pitch effects on maximum critical heat flux in evaporation and pool boiling heat transfer

ABSTRACTMolecular dynamics simulations were employed to investigate the effects of wettability (contact angle) and pitch on nanoscale evaporation and pool boiling heat transfer of a liquid argon thin film on a horizontal copper substrate topped with cubic nano-pillars. The liquid–solid potential was incrementally altered to vary the contact angle between hydrophilic (∼0°) and hydrophobic (∼127°), and the pitch (distance between nano-pillars) was varied between 21.7 and 106.6 Å to observe the resultant effect on boiling heat transfer enhancement. For each contact angle, the superheat was gradually increased to initiate nucleate boiling and eventually pass the critical heat flux (CHF) into the film boiling regime. The CHF increases with pitch, and tends to decrease substantially with increasing contact angle. A maximum overall heat flux of 1.59 × 108 W/m2 occurs at the largest pitch investigated (106.6 Å), and as the contact angle increases the superheat required to reach the CHF condition also increases. Finally, in certain cases of small pitch and large contact angle, the liquid film was seen to transition to a Cassie–Baxter state, which greatly hindered heat transfer.