Film Heat Transfer Coefficient Research Articles

Heat Exchanger Network Integration Using Diverse Pinch Point and Mathematical Programming

AbstractA design approach based on diverse pinch point and mathematical programming for optimal integration of heat exchanger networks (HENs) is described. The effect of the dissimilar film heat transfer coefficients on the area target is considered by different pinch and composite curves. The approach combines insightful pinch analysis and rigorous mathematical programming, which enable systematic and automatic designs. A software platform is developed for the design of HENs. An example illustrates the application of the proposed approach.

Investigation of the new corrugation pattern for low pressure plate condensers

The use of Plate Heat Exchangers (PHEs) as condensers for low pressure vapour is limited mostly because of relatively high pressure drop of condensing media. To overcome this obstacle the new principle of plate corrugation for low pressure plate-and-frame condensers is proposed. The proposed plate for condenser has the heat transfer area that consists of several differently corrugated sections along its length. Channels formed by such plates have the variable cross-section area. Pressure drop of single phase fluid in channels of such pattern was investigated experimentally and correlations for friction factor were obtained for each channel part formed by different sections of the plates. Based on analogy between heat and momentum transfer the equations to calculate film heat transfer coefficients in single phase flow and during condensation are proposed. To calculate condenser performance and local process parameters, the one dimensional mathematical model is used. Its validity is confirmed by comparison with literature data for condensation in PHE channels of investigated type. Case study results have shown the advantages of proposed approach. It enables to reduce considerably, about 40%, the pressure drop of condensing media, along with maintaining the level of heat transfer enhancement.

Condensation Heat Transfer Characteristics of a Separate Heat Pipe in Cool Storage Air‐Conditioning Systems

AbstractThe condensation characteristics model of the condensation part of a separate heat pipe in a cool storage air‐conditioning system is presented. In this model, the shear stress between gas and liquid two‐phase flow is considered. R134a is taken as working medium of the heat pipe. The liquid film thickness, shear stress, and heat transfer coefficient of the heat pipe are calculated. The effect of the shear stress on the film thickness and the variations of the heat transfer coefficient with heat pipe diameter and heat flux are analyzed. Influences of the heat pipe incline angle and heat flux on the heat transfer coefficient are also considered. The results indicate that the liquid film based on this model is thinner and the condensation heat transfer coefficient is larger than that based on the Nusselt model.

Discussions of some myths and concerned practices of film cooling research

This paper focuses on discussing four myths and concerned conventional practices of film cooling research guided by a series of computational simulations. The issues that have been discussed include: (a) the film cooling effectiveness ( φ) is given a constant (0.6) to calculate the heat flux ratio HFR = q ″ / q o ″ between film-cooled and no-film cases; (b) the adiabatic wall temperature is the driving temperature of film cooling; (c) the adiabatic film heat transfer coefficient can be obtained from an isoenergetic film experiment ( T j = T g); (d) using a heated surface can provide a simplified approach to simulate the film-cooling condition. The result shows that the adiabatic wall temperature is not always the driving temperature (i.e. T aw is not always larger than T w), but T aw does act well as the reference temperature in correctly predicting the heat flux direction and giving an always positive adiabatic film heat transfer coefficient. Using a constant value for φ is questionable and may lead to un-realistic or false HFR. The conventional equation of HFR was not theoretically exact and can lead to an error of up to 20%. A revised HFR equation is provided in this paper. The dominant energy passage in turbine airfoil film cooling is always from hot combustion gas flows into the airfoil; therefore, employing a heated wall as a boundary condition with hotter main stream flow and cold film injection to simulate the actual film cooling condition is found to be fundamentally questionable. A conjugate simulation that includes wall heat transfer and internal flow cooling shows that reversed heat transfer from blade to gas that gives a negative HFR is possible, due to the heat conduction from the downstream hotter region into the near film hole cooler region.

Thermal breakthrough analysis for determination of energy equilibrium data in a fluidized bed

The aim of this work was to predict energy equilibrium values in a bench-scale fluidized bed (FB: 105 × 200 mm), using a thermal breakthrough analysis (TBA). For this purpose, a simple “unsteady state” energy balance was proposed by harnessing dynamic model approach on the basis of heat exchange between the bed and the gas. To investigate thermal behavior of the bed, low temperature runs at different flow rates (5.2 ≤ Q 0, m 3 h − 1 ≤ 7.4) and heating rates (97 ≤ q, kJ h − 1 ≤ 765) were carried out. FB was heated by means of an electrical heater (10 × 50 mm) horizontally immersed into the bed particles for heating period and then the power input was terminated for cooling period. The bed temperatures ( T B) were continuously measured for obtaining thermal breakthrough curves for all periods. Temperature–time data were used for extracting bed-to-gas heat transfer film coefficients ( h BG) from linear forms of proposed model. The model was also employed for calculating amounts of shared energies by fluidized bed phases ( q y − q x ). A good agreement between experimental values and model values of T B was found. The results were thus confirmed by proposed model. The latter may be successfully used to predict energy equilibrium data for e.g. drying or combustion.

Heat transfer performance of lithium bromide solution in falling film generator

An experimental investigation of vertical in-tube falling film heat transfer with different heat fluxes and concentrations of lithium bromide solution were conducted. The experiments show that the heat transfer coefficient increases with the decrease of inlet concentration and significantly increase with heat flux increase. An experimental correlation of falling film heat transfer coefficient is obtained: h m = 129.7712 W in - 0.8058 q w 0.2422 Re - 0.0856 The comparison of falling film generator with immersed tube generator shows that the heat transfer coefficient is 4.37 times higher than that of immersed tube generator, which can significantly reduce the volume of the falling film generator. The volume of falling film generator is only 52.1% of the volume of immersed tube generator.

Thermal/structural analysis of radiators for heavy-duty trucks

A thermal/structural coupling approach is applied to analyze thermal performance and predict the thermal stress of a radiator for heavy-duty transportation cooling systems. Bench test and field test data show that non-uniform temperature gradient and dynamic pressure loads may induce large thermal stress on the radiator. A finite element analysis (FEA) tool is used to predict the strains and displacement of radiator based on the solid wall temperature, wall-based fluid film heat transfer coefficient and pressure drop. These are obtained from a computational fluid dynamics (CFD) simulation. A 3D simulation of turbulent flow and coupled heat transfer between the working fluids poses a major difficulty because the range of length scales involved in heavy-duty radiators varies from few millimeters of the fin pitch and/or tube cross-section to several meters for the overall size of the radiator. It is very computational expensive, if not impossible, to directly simulate the turbulent heat transfer between fins and the thermal boundary layer in each tube. In order to overcome the computational difficulties, a dual porous zone (DPZ) method is applied, in which fins in the air side and turbulators in the water side are treated as porous region . The parameters involved in the DPZ method are tuned based on experimental data in prior . A distinguished advantage of the porous medium method is its effectiveness of modeling wide-range characteristic scale problems. A parametric study of the impact of flow rate on the heat transfer coefficient is presented. The FEA results predict the maximum value of stress/strain and target locations for possible structural failure and the results obtained are consistent with experimental observations. The results demonstrate that the coupling thermal/structural analysis is a powerful tool applied to heavy-duty cooling product design to improve the radiator thermal performance, durability and reliability under rigid working environment.

E108 高圧タービン前縁フィルム冷却における冷却孔形状の効果(OS6 高温・高効率発電),動力エネルギーシステム部門20周年,次の20年への新展開)

This paper deals with the experimental and numerical studies of aero-thermal characteristics of film cooling for turbine blade leading edge having three differently hole geometries. The hole geometries used in this study are called typeA and typeB. The purpose of this study is to sort out the most suitable hole geometry. Mainstream Reynolds number based on the cylinder diameter was 86,000. The effect of blowing rates (BR) was studied of various blowing ratios of 1.0 and 2.0. Transient techniques using thermochromic liquid crystal is used to measure film effectiveness and heat transfer coefficient. In addition, Detailed investigation of film cooling for leading edge is carried out using CFD simulations. RANS applying a Shear Stress Transport turbulence model is addressed to solve thermal convection. Results show that the typeA provide better film cooling performance on BR=1.0 than typeB.

Kinetic Heat Sink

Conventional heat sinks for processors achieve improved heat transfer efficiency by impinging ambient air over stationary fins. Further improvement in the air film heat transfer coefficient through increased air speed becomes asymptotic and invariably results in undesirable acoustic noise. The Kinetic Heat Sink (KHS) is a novel concept that utilizes rotating metals fins to transfer heat to the ambient air. The configuration allows high-speed relative motion between surrounding air and fins with the potential of a high heat transfer coefficient at practical flow rates and rotational speeds. Improved efficiency of heat transfer is anticipated because of higher relative tangential velocities offered by the rotating fins and moving air compared to a conventional heat sink configuration. The need to move the fins relative to the stationary heat source, however, imposes a technical challenge. A highly conductive thin liquid film to couple the stationary heat source to the moving fins is required to demonstrate the merit of the KHS. The concept of the KHS has been evaluated in a number of prototypes that have been built and tested. The lessons learnt through a series of prototype KHS designs are summarized. The technical and performance challenges encountered during the feasibility testing of a KHS are then outlined.

Experimental investigation into the thermal-flow performance characteristics of an evaporative cooler

Abstract This study investigates the thermal-flow performance characteristics of an evaporative cooler. The derivation of the Poppe [1] and Merkel [2] analysis for evaporative coolers are presented and discussed. Performance tests were conducted on an evaporative cooler consisting of 15 tube rows with 38.1 mm outer diameter galvanized steel tubes arranged in a 76.2 mm triangular pattern. From the experimental results, correlations for the water film heat transfer coefficient, air–water mass transfer coefficient and air-side pressure drop are developed. The experimental tests show that the water film heat transfer coefficient is a function of the air mass velocity, deluge water mass velocity as well as the deluge water temperature, while the air–water mass transfer coefficient is a function of the air mass velocity and the deluge water mass velocity. It was found that the correlations obtained for the water film heat transfer coefficient and the air–water mass transfer coefficient compare well with the correlations given by Mizushina et al. [3] . Relatively little published information is available for predicting the air-side pressure drop across deluged tube bundles. The present study shows that the pressure drop across the bundle is a function of the air mass velocity and the deluge water mass velocity.

Theoretical and Experimental Analysis of Turbulent Liquid Film Flowing down a Vertical Surface

The purpose of the present study is to obtain a comprehension for the momentum and heat transfer developments in gravitational liquid film flow. Analytical study of stabilized heat transfer for turbulent film was performed. A calculation method of the local heat transfer coefficient for a turbulent film falling down a vertical convex surface was proposed. The dependence of heat flux variation upon the distance from the wetted surface has been established analytically. Experimental study of velocity profiles for turbulent liquid film flow in the entrance region is performed as well. Analysis of profiles allowed estimating the length of stabilization for turbulent film flow under different initial velocities.

Self-induced thermocapillary convection in film boiling heat transfer from a vertical surface in saturated conditions and viscous regimen

Self-induced thermocapillary convection and its significance with regard to film boiling heat transfer from a vertical saturated surface is discussed. Utilizing a simplified geometrical model, an analytical expression (multiplier factor) for the vapor film thickness and heat transfer coefficient corrected by thermocapillary phenomena was derived. The above equation is a new theoretical viewpoint for the enhancement in the heat transfer coefficient observed in the experimental data in the viscous regime and agree qualitatively with available experimental measurements made on R113 coolant.

A Practical Verification of Ambient Temperature Slide Method (ATS Method) during Retort Sterilization

In our previous report, we have proposed the Ambient Temperature Slide Method (ATS Method) as a rational way to estimate the central temperature of individually packed foods during retort sterilization. In this paper, we aimed to get new knowledge by specifically comparing ATS Method with simple One-Dimensional Finite-Difference Method (s-OFD Method) that was improved for the practical use of a conventional method using numerical calculation in order to solve the heat conduction equation (One-Dimensional Finite-Difference Method (OFD Method)). From the results of the experiments, it was clear that the Outside Film Heat-Transfer Coefficient h showed a smaller value than the expected one in the OFD Method and so surface heat transfer resistance is so large that it cannot be ignored. Because of this reason, it was thought to have been the obstacle for accurate estimating of the central temperature of foods. When we did parameter-fitting, ATS Method proved to be more than twice as good as the s-OFD Method in convergent accuracy and it was also good in convective foods as well as conductive foods. And, the F-value errors in simulation by using parameters obtained by parameter-fitting in the case of 4 food samples showed only one fifth of the value calculated on the s-OFD Method, it means that the convergent accuracy of ATS Method is five times better than the s-OFD Method. ATS Method showed approximately '1' in the ratio of thermal diffusivity for conductive foods. This result may indicate the possibility in simple theoretical calculation of parameter of heat transfer tau. From these results, ATS Method was thought to be several times better than the s-OFD method that is a conventional method and an improving one of OFD Method in estimating for the central temperature of foods.

Gas distribution and particle mobility in a model fluid-bed polymer recycling reactor

The effect on fluidisation quality of a number of sieve-tray and tuyere distributor designs has been investigated in a cold scale model of a proposed pyrolytic fluid-bed polymer recycling reactor. Fluidisation quality in a scale model of the proposed reactor design was assessed visually, by comparing bed pressure drop to solids weight per unit area, by measuring activity of individual nozzles and by the use of probes measuring film heat transfer coefficients at a number of locations across the bed. It was found that all the distributors gave good-quality fluidisation at fluidising velocities greater than three times the minimum-fluidising value, except in a dead zone near the distributor. This appears to support the standard models of fluid-bed distribution, however a more surprising observation was that uniform activity of all orifices did not necessarily mean that fluidisation was uniform elsewhere in the bed. These dead areas could trap incompletely reacted polymer particles and hence favour the accumulation of carbonised plastic agglomerates in the full-scale reactor bed. The effect of the configuration of internals on the dispersion coefficient of polymer particles was also appraised, by injecting a representative polymer sample, defluidising the bed, dissecting it and recording the position of sample particles. Experiments using Positron Emission Particle Tracking (PEPT) were conducted, giving insight into circulation patterns of the particles in the bed. Both dispersion experiments and PEPT demonstrated the presence of recirculation vortices in the free space between successive stacks of internals and extending into the stacks. These results suggest injecting the polymer at several locations, in order to make use of these vortices for dispersion without overloading them during the initial melting stage.

Modeling Heat Loss during Self-Heating Composting Based on Combined Fluid Film Theory and Boundary Layer Concepts

The overall heat-transfer coefficients ( U-values) developed during the self-heating composting of mixed vegetables and chicken manure were determined based on a combined fluid film theory and thermal boundary layer concept. The heat flow was modeled into four heat transfer components. The heat transfer mechanisms have been modeled as comprising two conductive heat fluxes across the compost matrix and compost reactor walls, one heat flux across the film of water condensing on the inner reactor surface and a convective heat flux due to free convection on the outside surface of the reactor walls. Individual film heat-transfer coefficients have been calculated using a series of dimensionless correlations grouping the outer reactor surface temperature, the Nusselt (Nu), Prandtl ( Pr), Grashof ( Gr) and Rayleigh ( Ra) numbers. Gr has varied between 5.11 × 10^6 and 3.71 × 10^9, Pr from 0.692 to 0.712, and Ra from 3.64 × 10^6 to 2.56 × 10^9. Four different equations have been used to evaluate the heat transfer coefficients characterizing the heat flow due to free convection. The maximum U-values have been found to vary between 0.368 and 0.387 W.m^-2·K^-1, and the minimum U-values have ranged from 0.255 to 0.288 W.m^-2·K^-1. A 4-parameter Weibull model was found to describe the variation of U-value with compost matrix temperature reliably with R^2 = 0.9999.

A Novel Antivortex Turbine Film-Cooling Hole Concept

A novel turbine film-cooling hole shape has been conceived and designed at NASA Glenn Research Center. This “antivortex” design is unique in that it requires only easily machinable round holes, unlike shaped film-cooling holes and other advanced concepts. The hole design is intended to counteract the detrimental vorticity associated with standard circular cross-section film-cooling holes. This vorticity typically entrains hot freestream gas and is associated with jet separation from the turbine blade surface. The antivortex film-cooling hole concept has been modeled computationally for a single row of 30 deg angled holes on a flat surface using the 3D Navier–Stokes solver GLENN-HT. A blowing ratio of 1.0 and density ratios of 1.05 and 2.0 are studied. Both film effectiveness and heat transfer coefficient values are computed and compared to standard round hole cases for the same blowing rates. A net heat flux reduction is also determined using both the film effectiveness and heat transfer coefficient values to ascertain the overall effectiveness of the concept. An improvement in film effectiveness of about 0.2 and in net heat flux reduction of about 0.2 is demonstrated for the antivortex concept compared to the standard round hole for both blowing ratios. Detailed flow visualization shows that as expected, the design counteracts the detrimental vorticity of the round hole flow, allowing it to remain attached to the surface.

On the evaluation of the global heat transfer coefficient in cutting

The use of numerical simulations for investigating machining processes is remarkably increasing because of the simulation cost is lower than the experiments and the possibility to analyze local variables such as pressures, strains, and temperatures is allowable. Process simulation is very hard from a computational point of view, since it frequently requires remeshing phases and very small time steps. As a consequence, the simulated cutting time is usually of the order of few milliseconds and no steady cutting conditions are generally achieved, at least as far as thermal conditions are concerned. Therefore, nowadays numerical prediction of cutting temperatures cannot be considered fully reliable. In the paper this issue was taken into account: a mixed Lagrangian-Eulerian numerical approach was utilized and the global heat transfer (film) coefficient at the tool-chip interface was derived through an inverse approach. Finally, the dependence of the film coefficient on pressure and temperature on the rake face was investigated.

Targets for Heat Exchanger Network Synthesis with Different Heat Transfer Coefficients and Non-uniform Exchanger Specifications

In the synthesis of heat exchanger networks, the estimation of minimum area requirements plays an important role since it allows for the prediction of investment requirements ahead of design. In this work, a method to calculate the area target and capital cost target for the synthesis of heat exchanger networks is presented. The method is suitable for cases in which there are significant differences in the film heat transfer coefficients among process streams and when different cost laws for heat exchangers are considered. The area targeting algorithm is based on the concept of Δ T-contribution for each stream. Two examples are shown to illustrate the application of the proposed method.

Local Flow and Heat Transfer Behaviours in V-L-S Fluidized Bed Evaporator

The fluidized bed evaporator is a subject of considerable interest and can be employed in the process industries. However, the V-L-S flow behavior and heat transfer characteristics are still not well understood. In this work, a vertical fluidized bed evaporator with external natural-circulation flow boiling was established to investigate the local V-L-S flow behaviour and heat transfer characteristics and to reveal the influence of flow on heat transfer performance. It consists of a heated tube with inner diameter of 3.8 × 10 −2 m and height of 1.4 m and a circulating tube with the same inner diameter and height of 0.92 m. Both tubes are made of quartz glass plated with transparent electrically heating film. The solid particles added to the evaporator are glass, ceramic, Teflon™ and poly-formaldehyde beads or cylinders with the diameter ranging from 1.8 × 10 −3 to 4 × 10 −3 m. The particle volume changes from 0 to 1 × 10 −3 m 3 and heat flux varies from 5 × 10 3 to 1.2 × 10 4 W m −2. The local velocity and holdup of solid particles, flow region transition, length of the V-L-S flow boiling, fluid circulating velocity and pressure drop in the V-L-S fluidized bed evaporator were visually investigated with CCD measuring technique. The main results on flow are as following. Axial solid holdup in heated tube of the evaporator decreases obviously for middle-density particle systems against the direction of the gravity. Three flow regions can be found in heated tube of the evaporator: L-S region, transition region and V-L-S region, and in transition region, the radial profile of solid holdups is relatively uniform. The increase of particle volume enlarges the length of V-L-S region and pressure drop, while decreases the circulating velocity of fluid mixture. The average heat transfer film coefficients of V-L-S flow boiling were estimated and a dimensionless correlation was obtained based on 120 sets of experimental data with a maximum relative deviation of 10.8%. The axial variation of the heat transfer coefficients has close relation to the axial distribution of the solid holdups.

Characteristics of Flow and Heat Transfer in a Tube Bundle Evaporator with a Vapour–Liquid–Solid Flow

In this paper, a tube bundle experimental evaporator with a specific vapour–liquid–solid (V-L-S) flow boiling system with external natural circulating is established to imitate and investigate a typical three-phase flow and boiling heat-transfer characteristics in order to reveal the mechanism of this apparatus. It is shown that under the ranges of the experimental parameters tested, the average heat transfer film coefficient h fb increases with increasing of temperature difference between the inside surface of tube wall and the boiling fluid. Lowering holdup of the solid particles enhances the heat transfer process. Meanwhile, the average heat transfer film coefficient h in each tube was found to be quite different for different tubes. The addition of solid particles enlarges this difference. The circulating velocity in the tube in V-L-S flow boiling was found to increase with the increase of the overall driving force of heat transfer or temperature difference between the steam and the boiling fluid, and it is higher than that of the V-L flow boiling. Opposite is true for the pressure drop in the tube in V-L-S flow boiling. There is no obvious effect of circulating velocity but there is a clear distinction in the pressure drops when varying the solid holdups. These results may offer some useful data sets to design, scale-up of this type of evaporators.