Film Heat Transfer Research Articles

Heat transfer of a Carreau fluid in a thin elastic film over an unsteady stretching sheet

An analysis is carried out to investigate the influence of the magnetic field over Carreau fluid flow and heat transfer in a liquid film on an unsteady stretching surface. The governing equations are converted to a system of boundary value problem (BVP) by using the similarity transforms. The differential equations are solved numerically using the modified Laplace decomposition method (MLDM) and the obtained results are compared with the existing schemes. An excellent agreement between them illustrates the accuracy of MLDM to the suggested model. The influence of the dimensionless governing parameters like magnetic field number, Prandtl number, unsteadiness parameter, Weissenberg parameter, and radiative number on velocity and temperature profiles is discussed in tabular form and shown through illustrative graphs.

Long-range polaritonic heat conduction in asymmetric surrounding media

Surface Phonon Polaritons (SPhPs) as an evanescent electromagnetic surface wave supports long propagation which readily surpasses the mean free path of classical heat carriers, e.g., phonons and electrons in solids. SPhPs have emerged as a promising candidate to dominate heat transfer in thin films. Polaritonic heat transfer has two distinct advantages: superior thermal conduction and a wide range of manipulation. Here, we study the upper limit of the thermal conductivity mediated by long-range polaritons in asymmetric surrounding media, where its surface effect overwhelms the volumetric one. The thin film structure strengthens the interactions of two surface waves at the top and bottom surfaces, and the asymmetric surrounding media makes an evanescent surface wave to further penetrate the free space with a higher refractive index, but it requires a fine tuning of asymmetric permittivity of the surrounding media to reach the upper limit of energy transmission efficiency near to the modal cut-off, where the transverse wavevector becomes zero. Both analytical and numerical simulations were introduced to investigate dispersion in asymmetric surrounding media and to model the thermal conductivity of glass thin films. Anomalously high thermal conductivity of 248 W/m-K was achieved with a 50 nm thick SiO2 film in asymmetric surrounding media, yet subtly dissimilar.

Reduction of heat transfer and improvement of film cooling effect on squealer tip with multi-rib design concept

Reduction of heat transfer and improvement of film cooling effect on squealer tip with multi-rib design concept

Experimental and numerical study on the falling film flow process on the outer wall of dome cylinder

The passive containment cooling system (PCCS) is crucial for ensuring the safety of nuclear power plants, and the thickness and temperature of the falling film on its outer wall are important parameters that determine the heat and mass transfer performances. In this paper, a compact liquid film thickness and temperature simultaneous measurement system was developed to provide a measurement method, which is based on the absorption spectroscopy technology, and then the measurement accuracy was verified by using a calibration tool. The results showed that the mean relative error of the liquid film thickness and temperature measurements from the known values were about 3.4% and 0.4%, respectively. On this basis, this system was used to study the falling film process on the outer wall of the dome cylinder analogous to the containment under different working conditions and combined with numerical simulation for comparison. The results showed that the two methods have good consistency. The variation of the liquid film heat transfer coefficient for various operating conditions was further analyzed. It was found that the average thickness of the falling film is positively correlated with the liquid flow rate, but negatively correlated with the temperature and heat transfer coefficient. In summary, this system can accurately study the falling film process on the outer wall of the dome cylinder, which is expected to accurately analyze the heat and mass transfer process in the PCCS and provide data support for its design and optimization.

Heat transfer in MHD thin film flow with concentration using lie point symmetry approach

Heat and mass transfer in a viscous film on an unsteady stretching surface in the presence of a variable magnetic field is investigated using Lie symmetry analysis. Combination of translational and scaling Lie point symmetries is used to obtain invariants of the model. The deduced invariants provide a new generalized class of similarity transformations that have not been used before. These transformations restructure the governing problem in a system of eight-parameter nonlinear ordinary differential equations. Analytic series solutions are obtained for the resulting system of equations for different values of these parameters and are illustrated graphically. We found that coefficients of the translational symmetries do not have any effect on the solutions however, coefficients of the scaling symmetries significantly affect the variation of temperature and concentration distribution across the fluid film and help in controlling the heat and mass transfer rates. Also, increasing the value of unsteadiness and magnetic parameter decreases the film thickness and promotes a uniform temperature and concentration across film thickness. Furthermore, we found that at the lower values of Prandtl and Schmidt number, diffusion dominates the heat and mass transfer while at the higher values advection dominates the heat and mass transfer.

A numerical study on thin film flow and heat transfer enhancement for copper nanoparticles dispersed in ethylene glycol

Abstract The current study examines thin film flow and heat transfer phenomena with some additional effects such as magnetohydrodynamic, viscous dissipation, and slip condition over unsteady radially stretching surfaces for various shapes of copper ( Cu ) \left({\rm{Cu}}) nanoparticles dispersed in ethylene glycol ( EG ) \left({\rm{EG}}) . The effective thermal conductivity of a nanofluid made of Cu nanometer-sized particles distributed in EG {\rm{EG}} is significantly higher than that of pure EG. Partial differential equations are transformed into ordinary differential equations using the proper transformations. An effective convergent technique (i.e., BVP4C) is used to compute the solutions of nonlinear systems. MATLAB software is used to perform the calculations. The effect of numerous emerging physical characteristics on temperature and velocity, such as unsteadiness parameter ( S ) \hspace{ 1em}\left(S) , slip parameter ( K ) \left(K) , Hartmann number ( M ) \left(M) , solid volume fraction ( ϕ ) (\phi ) , and Eckert number ( EC ) \left({\rm{EC}}) is investigated and illustrated graphically. The physical quantities, such as the skin friction coefficient and the Nusselt number, are calculated, described, and displayed in tabular form. It is observed that blade-shaped Cu nanoparticles had the lowest surface drag, highest heat transfer rate, and minimum film thickness compared to the brick and cylinder-shaped nanoparticles. According to our detailed investigation blade-shaped Cu {\rm{Cu}} nanoparticle is the most suited solution for manufacturing unsteady radially stretching modules.

Heat Transfer in a Falling Liquid Film of Freon R21 on an Array of Horizontal Tubes with Modified MAO Coatings

Heat Transfer in a Falling Liquid Film of Freon R21 on an Array of Horizontal Tubes with Modified MAO Coatings

Numerical simulation of condensation heat transfer characteristics of R134a at high vapor velocity under different angles

Heat pipe is widely used as an efficient built-in heat exchanger for thermal management of high pulse and heat flux power supplies. Enhanced heat dissipation at the condensing side is an important issue in heat pipe research, where vapor velocity is an essential factor affecting the condensing heat transfer characteristics. This study adopted the CFD method to numerically simulate the condensation heat transfer of R134a flowing in a smooth pipe. The flow condensation heat transfer characteristics, such as film thickness and heat transfer coefficient (HTC), were investigated under vapor velocities v = 20–100 m/s and tube inclinations β =-90–90° Geometric and numerical heat transfer modeling is established in high vapor velocity flow condensation. The error of numerical simulation results and model validation is 10%. The HTC increases rapidly with the change in vapor velocity under 60 m/s, increasing the rate to 0.4. When the vapor velocity exceeds 60 m/s, the growth trend slows to 0.15, 37.5% of the original increase. At high vapor velocities above 60 m/s, the effect of pipe inclination on condensation heat transfer coefficient can be almost negligible. A modified empirical correlation suitable for high vapor velocities is proposed, which has higher predictive accuracy when the vapor velocity exceeds 80 m/s.

Multiple Lie symmetry solutions for effects of viscous on magnetohydrodynamic flow and heat transfer in non-Newtonian thin film

Abstract Numerous flow and heat transfer studies have relied on the construction of similarity transformations which map the nonlinear partial differential equations (PDEs) describing the flow and heat transfer, to ordinary differential equations (ODEs). For these reduced equations, one finds multiple analytic and approximate solution procedures as compared to the flow PDEs. Here, we aim at constructing multiple classes of similarity transformations that are different from those already existing in the literature. We adopt the Lie symmetry method to derive these new similarity transformations which reveal new classes of ODEs corresponding to flow equations when applied to them. With these multiple classes of similarity transformations, one finds multiple reductions in the flow PDEs to ODEs. On solving these ODEs analytically or numerically, we obtain different kinds of flow and heat transfer patterns that help in determining optimized solutions in accordance with the physical requirements of a problem. For the said purpose, we derive Lie point symmetries for the magnetohydrodynamic Casson fluid flow and heat transfer in a thin film on an unsteady stretching sheet with viscous dissipation. Linear combinations of these Lie symmetries that are again the Lie symmetries of the flow model are employed here to construct new similarity transformations. We derive multiple Lie similarity transformations through the proposed procedure which lead us to more than one class of reduced ODEs obtained by applying the deduced transformations. We analyze the flow and heat transfer by deriving analytic solutions for the obtained classes of systems of ODEs using the homotopy analysis method. Magnetic parameters and viscous dissipation influences on the flow and heat transports are investigated and presented in graphical and tabulated formats.

Experimental investigation and modeling of falling film heat transfer on partial dry-out condition

Falling film heat transfer has been applied for decay heat removal in more and more nuclear reactors, in which the coolant feed rate is set small to prolong the effective cooling time during accident conditions. Previous works have shown that the cooling surface is under partial dry-out condition for most of the time during the accident, while the existing falling film experiments are mainly focused on fully-wetted condition. In this paper, falling film experiments were conducted on a horizontal plain copper tube within 0.019–0.099 kg/m·s coolant flow rate, 0–164.1 kW/m2 heat flux, and dry-out condition is emphasized. In the experiments, it is found that the overall liquid coverage rate measured by infrared equipment was between 45.2 % and 100 %, and the dry-out process is recorded and analyzed as well. Moreover, HTC (Heat Transfer Coefficient) at wetting area is defined to characterize the heat transfer on the surface covered by liquid and is obtained based on a three-dimensional heat conduction model of test tube. Boiling HTCs on falling film are found 10 % approximately higher than that in pool boiling condition. The difference is attribute to the different mechanism of bubble departure and bubble growth environment, with which the falling film condition is found to result in a 1–3 times bubble departure diameter and a 0.85 times bubble growth rate compared with pool boiling condition. Finally, a general falling film heat transfer model is developed based on the conservation equations with consideration of liquid coverage, gas–liquid interface evaporation and liquid sub-cooling. The model shows effective for falling film heat transfer prediction on both fully-wetted and dry-out, convection and boiling conditions with different working fluids through the validation with a number of experimental data. The findings in this work could be helpful for the design and analysis of nuclear cooling system, as well as the falling film evaporation system.

Experimental decoupled-analysis of overall cooling effectiveness for a turbine endwall with internal and external cooling configurations

High-efficient composite cooling structures that are comprised of external and internal cooling techniques, are one of the key technologies to guarantee the aerothermal performance and reliability of a turbine vane, especially the endwall region due to unique complex flow structures. It is thereby critical to understand the flow mechanism and conjugate heat transfer characteristics in this region for further optimal design. In this paper, a representative endwall model which matches non-dimensional parameters to real turbine operating conditions is investigated. Thermal protection for the endwall is provided by internal jet impingement and external purge flow and discrete hole injection cooling. Comprehensive measurements of overall cooling effectiveness over the endwall, total pressure loss coefficient, and non-dimensional temperature at the cascade exit are performed by using an infrared (IR) thermography technique, a five-hole probe, and a temperature probe. Through decoupled analysis, heat transfer and flow characteristics of film cooling only, both jet impingement and film cooling, upstream purge cooling only, and combined cooling are analyzed, respectively, and their optimal ranges of cooling air are obtained. The measurements reveal that with film cooling only, only the regions around the film holes are covered. Using both film and jet impingement cooling, cooling coverage is expanded, and cooling effectiveness is much higher with a more uniform distribution pattern. Purge flow achieves thermal protection for upstream regions of endwalls, and the combined cooling scheme that involves all cooling sources provides the best cooling performance. Globally, those cooling schemes have no obvious effects on overall cascade aerodynamic performance but affect the development and distribution of secondary flows.

Flow and heat transfer analysis of the microfluidic thermal camouflage film based on bionic structure

With the rapid advancement of infrared (IR) detection techniques, the importance of thermal camouflage technology has become increasingly prominent. Inspired by spider webs and honeycombs, microfluidic thermal camouflage films based on two bionic structures are proposed to IR cloak targets by circulating microfluidics in the film. The film displays an adaptive thermal camouflage effect by automatically adjusting the microfluid temperature. Microcavity and microchannel are used as the main internal structure of camouflage film. To compare the thermal camouflage performance of two corresponding bionic structures, heat transfer and fluid flow mechanisms are analyzed. The structure and working principle of the film are introduced, and the testing system is developed to explore the flow and heat transfer characteristics of the bionic spider web micro-channel film and honeycomb cell film. The comparison reveals that the honeycomb cell film's temperature distribution uniformity and heat transfer performance are better than the spider web micro-channel film's, which is suitable for thermal camouflage. Considering the influence of the liquid low-velocity stagnation zone on the camouflage performance, rounded corners of the honeycomb cell are designed to improve the fluid flow properties, and the relationship between the corner radius and the heat transfer characteristics is further investigated. Finally, the honeycomb cell film's parameters are optimized by orthogonal tests, and the optimal solution enhances the thermal camouflage performance. The results show that the film has a good stealth effect, which is expected to motivate the further application of bionic structures in the military field.

Identification of the ultra-short laser parameters during irradiation of thin metal films using the interval lattice Boltzmann method and evolutionary algorithm

The article presents the one-dimensional numerical modelling of heat transfer in thin metal films irradiated by ultra-short laser pulses. In the mathematical description the relaxation times and the boundary conditions for phonons and electrons are given as interval numbers. The direct problem has been solved by means of the interval lattice Boltzmann method using the rules of directed interval arithmetic, while at the stage of the inverse problem solution, the evolutionary algorithm is applied. On the stage of inverse problems investigations, the evolutionary algorithm coupled with the interval lattice Boltzmann method is used.

Experimental and numerical studies on the spray cooling of deionized water in non-boiling regime

High-efficiency spray cooling technology has received increase of interest for high heatflux applications. Experimental and numerical investigations on spray heat transfer in single-phase deionized water are reported in this paper. The 3D numerical model is established based on Discrete Phase Model and Lagrange Wall Film model, comprehensively considering the exchange of mass, momentum and energy between the air (continuous phase) and water droplets (discrete phase), and the heat transfer in liquid film at the heated surface. PDA is adopted to measure Sauter Mean Diameter (SMD) of near-wall droplets. The influences of mass flow rates, nozzle diameters and spray heights on spray heat transfer characteristics are discussed. Results indicate that increasing mass flow rate can reduce SMD due to high We number and improve heat transfer coefficient. Liquid-film thickness is high at either low or high mass flow rate, but has minor influence on cooling performance. The optimal nozzle diameter is 0.6 mm rather than the smaller or higher diameter because the spray area is similar to the heated area. Similar reason is for the optimal spray height of 20 mm.

Lie symmetry and exact homotopic solutions of a non-linear double-diffusion problem

The Lie symmetry method is applied, and exact homotopic solutions of a non-linear double-diffusion problem are obtained. Additionally, we derived Lie point symmetries and corresponding transformations for equations representing heat and mass transfer in a thin liquid film over an unsteady stretching surface, using MAPLE. We used these symmetries to construct new (Lie) similarity transformations that are different from those that are commonly used for flow and mass transfer problems. These new (Lie) similarity transformations map the partial differential equations of a mathematical model under consideration to ordinary differential equations along with boundary conditions. Lie similarity transformations are shown to lead to new solutions for the considered flow problem. These solutions are obtained using the homotopy analysis method to analytically solve the ordinary differential equations that resulted from the reduction of considered flow equations through Lie similarity transformations. With the aid of these solutions, effects of various parameters on the flow and heat transfer are discussed and presented graphically in this study.

Influence of horizontal tube bundle arrangement on flow and heat transfer properties of oily wastewater spray falling film

Recently, spray heat exchangers have become an important technology for recovering heat energy from oily wastewater in sewage source heat pumps (SSHP), which makes it attractive to study the heat transfer characteristics and its enhancement. It’s indicated in previous studies the arrangement of the tube bundle is the key influential factor for falling film flow and heat transfer. Considering the specific physical properties of oily wastewater, it’s necessary to study the role of oily content in bundle arrangement influences. Therefore, in this paper, inline tube bundles and triangle tube bundles are designed, and falling film of oily wastewater over these tubes were numerically investigated. The CFD model was established with VOF method to capture the gas–liquid interface, and the influence of different tube arrangement on the flow pattern and heat transfer effect of the liquid film under the flow parameters and structural parameters are analyzed. It is found that there is a significant difference in the falling film flow outside the horizontal tube between the triangle and inline tube bundle arrangement. The film flow of the triangle arrangement is more unstable, with liquid bridges between the liquid films, and the velocity inside the liquid film is relatively higher. Obvious variations of local heat transfer coefficient (HTC) during θ = 0°-30° were observed in both types of tube bundle arrangements. The average HTC of triangle arrangement is higher than that of the inline arrangement by 8 % to 16.8 % under various operating conditions.

Experiment on liquid film flow and heat transfer of laminar liquid jet impacting on cylindrical surface

Laminar liquid jet impact has good heat transfer performance with low impact stress on targets. In order to explore the liquid film flow and heat transfer under laminar liquid jet impact on a cylindrical surface, first the flow of a liquid film was analyzed visually; then, the local convective heat transfer characteristics on the cylindrical surface at different impact heights and outlet Reynolds numbers (Re) were obtained by a combination of direct measurement and numerical simulation, followed by a comparative analysis with continuous droplets impacting on the cylindrical surface. The results show that according to flow behavior of the liquid film along the circumferential direction, circumference can be classified into stagnation, thin liquid film, hydraulic jump, stable flow path, and dripping regions. Local convective heat transfer coefficient first drops and subsequently increases marginally along the circumferential direction, while decreasing monotonically along the axial direction. The effect of impact height and outlet Re on local convective heat transfer coefficient is manifested mainly in stagnation, thin liquid film, and hydraulic jump regions. For outlet Re = 984, as impact height rises to a certain degree, there are apparent enhancements of the liquid film perturbation and convective heat transfer performance. Finally, the local Nusselt number correlations in different circumferential regions were proposed.

Numerical study of the cross-vapor stream effect on falling film heat transfer performance over a horizontal tube

Vapor shearing can cause a falling film (FF) evaporator to exhibit various unusual phenomena. Under a cross-vapor stream, numerical simulations were conducted to examine the FF heat transfer performance outside a horizontal tube. The liquid film spreading, temperature fields, as well as time-averaged peripheral and axial, peripheral- and axial-averaged, and average heat transfer coefficients (HTCs) were presented, respectively. According to the results, we found that: (1) the cross-vapor stream produces deflections in liquid film spreading; (2) several zones of the tube surface have distinct distributions of HTCs, velocity vector fields, and liquid film thicknesses (LFT); (3) the cross-vapor causes apparent asymmetry for the axis-averaged peripheral HTC, especially at the higher vapor velocities; (4) the profiles of time-averaged peripheral and axial HTC on the upwind side are quite different from the one on the downwind side due to the cross-vapor; (5) within the current range of liquid flow rates, three trends of average HTCs can be observed with increasing vapor velocity; and (6) larger liquid flow rates can tolerate higher cross-vapor velocities to guarantee a better FF heat transfer performance.

Investigation on wave morphology and temperature distribution of falling liquid film in a vertical tube

The waves of the falling liquid film have a strong interaction with its temperature, and the flow and heat transfer mechanisms contained therein are of great significance for optimizing industrial equipment and improving energy efficiency. In this paper, the thickness and temperature field of the liquid film in a vertical tube were measured simultaneously based on planar laser-induced fluorescence (PLIF). The wave morphology, as well as the temperature distribution within the liquid film, was analyzed by high-speed imaging visualization and the film temperature extraction method, respectively. With the increase of the Reynolds number, an evolution of the wave morphology from ripples to high-frequency disturbance waves and finally to solitary waves was observed. The method of Dynamic Mode Decomposition (DMD) was developed to reconstruct the flow field and calculate the characteristic frequencies in the flow, from which the wave features under different wave morphologies were revealed. It was found that the temperature variation of the liquid film at the gas-liquid interface was accelerated by the waves from the results of the temperature distribution of the liquid film. Besides, the low-temperature micro clusters observed in the solitary waves can be considered as one mechanism of the enhancement of heat transfer in the liquid film.

A Scalable Heat Pump Film with Zero Energy Consumption.

Radiative cooling is an effective technology with zero energy consumption to alleviate climate warming and combat the urban heat island effect. At present, researchers often use foam boxes to isolate non-radiant heat exchange between the cooler and the environment through experiments, so as to achieve maximum cooling power. In practice, however, there are challenges in setting up foam boxes on a large scale, resulting in coolers that can be cooled below ambient only under low convection conditions. Based on polymer materials and nano-zinc oxide (nano-ZnO, refractive index > 2, the peak equivalent spherical diameter 500 nm), the manufacturing process of heat pump film (HPF) was proposed. The HPF (4.1 mm thick) consists of polyethylene (PE) bubble film (heat transfer coefficient 0.04 W/m/K, 4 mm thick) and Ethylene-1-octene copolymer (POE) cured nano-ZnO (solar reflectance ≈94% at 0.075 mm thick). Covering with HPF, the object achieves 7.15 °C decreasing in normal natural environment and 3.68 °C even under certain circumstances with high surface convective heat transfer (56.9 W/m2/K). HPF has advantages of cooling the covered object, certain strength (1.45 Mpa), scalable manufacturing with low cost, hydrophobic characteristics (the water contact angle, 150.6°), and meeting the basic requirements of various application scenarios.