Maximum Total Heat Transfer Research Articles

Analysis of the influence of deflector shape on heat transfer rate in glass tempering process

To solve the problem of uneven cold air flow during the tempering process, which causes glass self-detonation, distortion, and excessive fan energy consumption. Three deflector-shaped structures are proposed to enhance the cooling efficiency of the glass. Based on the combination of fluent numerical analysis and experiment, this study gives an in-depth examination of the heat transference of glass within a quenching system. The influence of deflector shapes on the heat transfer efficiency of glass is explored. The three deflectors were compared with traditional deflectors in terms of temperature, fluid flow, total heat transfer rate, and heat transfer coefficient. The simulation results agree well with the experimental data, and the established model of the glass quenching system is reliable. The rectangular deflectors reduce the glass base temperature by 3.03 K compared to traditional deflectors. Furthermore, when the rectangular deflectors are utilized, the maximum heat transfer coefficient of the glass can be up to 96.4 W/(m2·K) and the maximum total heat transfer can be up to 2622.82 W. In comparison to the traditional deflector shape, the maximum heat transfer coefficient of the designed rectangular deflectors may be enhanced by 0.58% and the maximum total heat transfer by 0.37%. The proposed design provides a suitable solution for the optimization of the tempering process as well as for the energy savings and efficiency of tempering equipment.

Numerical study on reaction and heat transfer of supercritical aviation kerosene with pyrolysis and steam reforming in a corrugated cooling channel

Coking and heat transfer deterioration of supercritical aviation kerosene pose grave challenges to regenerative cooling of scramjet engines. In the present study, reaction and heat transfer characteristics of supercritical China aviation kerosene No. 3, RP-3 undergoing simultaneous pyrolysis and steam reforming reactions in a corrugated channel are numerically studied. Compared with reacting flow of RP-3 in the smooth channel, temperature and coke deposition are much lower in majority part of the corrugated channel. The maximum temperature difference between the smooth and corrugated channels is up to 300K. The maximum heat transfer coefficient in the corrugated channel is almost three times of that in the smooth channel under the same condition. However, low-velocity and high-temperature zones are formed near the corrugated micro-structures and cause coke accumulation and heat transfer deterioration along the flow direction of the corrugated channel. With the increase of wall heat flux, both temperature and velocity increase significantly in the corrugated channel. The conversion of RP-3 and formation of coke are also enhanced with increasing wall heat flux. Moreover, the total heat transfer coefficient first increases and then decreases with wall heat flux. The maximum total heat transfer coefficient increases from 4000W/m2∙K to 18,000W/m2∙K with wall heat flux increasing from 0.3MW/m2 to 1.8MW/m2. Periodic wavy structures are found at the interface between the corrugated micro-structure and the bulk flow, which is pronounced when wall heat flux is 1.3MW/m2 or above. With the increase of flow time, local turbulent kinetic energy decreases due to the interaction between the corrugated micro-structures and the wavy structures. The interaction between the corrugated structures and complex reaction and heat transfer leads to intersection of the distributions of temperature and coke in the smooth and the corrugated channels. An empirical heat transfer correlation formula is obtained as Nu=0.1Re0.65Pr1.94 for RP-3 reacting flow in the corrugated channel. The study provides better insight into interaction mechanism between chemically reacting flow and micro corrugated structures.

Experimental Investigations on Performance Analysis of a Wickless Thermosiphon Heat Pipe With Two Heat Sources and Multiple Branches

Abstract In an electronic circuit of laptops, supercomputers with multiple central processing units, spacecraft etc., it is required to arrange the cooling system for multiple heat loads in the smallest possible space in view of power-saving opportunities. In the present study, the experimental investigations are carried out on a wickless multi-branch heat pipe in gravity-assisted mode with two evaporators and one condenser on each of the individual branches. The start-up and dynamic characteristics were studied with different filling ratios (range, 40–70%), with equal heat loads (range, 0–200 W) and unequal heat loads (range, 0–100 W) on evaporators. The results are analyzed in terms of temperature variation in axial direction, thermal resistance, and heat transfer coefficient for a multi-branch thermosiphon heat pipe (MBTHP). It was found that the optimal filling ratio depends on the applied heat load under tested conditions. In a thermosiphon mode, the heat pipe was capable of transporting a maximum heat load of 210 W and maximum heat flux of 20.31 W/cm2 with the maximum evaporator temperature lower than 100 °C. The minimum wickless thermal resistance of heat pipe was found to be 0.21 °C/W at 50% filling ratio and 160 W and maximum total heat transfer coefficient was found as 6.33 k W/m2 °C.

THE EFFECT OF ASPECT RATIO OF ELLIPTICAL COOLING HOLES ON THERMAL CHARACTERISTICS OF GAS TURBINE BLADE

In advanced gas turbine, the improvement of the thermal efficiency and power output is required. Among these improvements, which still getting attention considerably, increasing turbine inlet temperature that may exceed the melting point of the blade material and to avoid that internal blade cooling technique is incorporated. The common traditional cross-sectional area of the cooling channels is circular. In this paper, a modification regarding the shape of cooling hole is suggested to enhance the heat transfer between the coolant in cooling serpentine and hot gases from the combustion chamber and thus minimizing the severe temperature usually occurs in the gas turbine blade especially at first row of the rotor. The proposed shape is elliptical and to discuss different scenarios of cooling schemes, the aspect ratio of ellipse axes and the distribution of whole holes are also analyzed to figure out the best results in terms of temperature distribution, heat transferred and cooling effectiveness of each model. The three new models consist of cooling holes of (5, 6 and 7) located at the center of the curvature line of blade. Results were compared with those obtained using the model of the GE company (model-1) which is already installed in (AL-DIWANIAH GAS TURBINE POWER PLANT). It were analyzed on the basis of real data, such as geometry profile, total cooling area, and boundary conditions. Results have been discussed and it is found that model-4 is the optimum solution, because of maximum total heat transfer rate and reduction in the blade trailing edge temperature about 24% are attained in this model compared to model-1, and it is found increase number of cooling holes in the blade leads to lowers the blade trailing edge temperature, this depends on the design location of this holes on the blade surface. Meanwhile, the optimum ratio of the model-4 in the reduction of maximum temperature is (a / b = 2) by the dimensions of (a = 2 & b = 1). Steady state thermal analysis is carried out using CFD, Inconel 718 alloy selection as material and air is used as a coolant as the real condition of.

Optimization of Heat Transfer and Pressure Drop in Axially Finned Staggered Tube Banks

Numerical optimization of heat and fluid flow over staggered tube banks are investigated for both unfinned and axially finned tube banks. Ansys Fluent software is utilized for numerical computations. To find maximum total heat transfer and minimum volume of tube bank for the given mass flow rate, allowable pressure drop and tube bank effectiveness; A Multi-objective Genetic Algorithm II is used. Optimum designs are given on tabular forms and variations of total heat transfer with total volume are presented. Moreover, effects of transversal and longitudinal pitches on objective functions are graphically demonstrated. In order to confirm reliability of the numerical studies, unfinned staggered tube bank configuration is compared with experimental studies in literature. As a result of optimization studies, axial fin usage results in 42.7% decrement in volume of staggered tube bank with respect to unfinned case, while heat transfer values are equal with maximum obtainable heat transfer rate for the given optimization problem.

Pilot-scale study on flue gas moisture recovery in a coal-fired power plant

The exhaust gas from coal-fired power plants installed with desulfurization tower contains a large amount of water vapor, recovering water from flue gas for recycling is of great significance in water saving and sustainable development. In this work, a transport membrane condenser was assembled by using the 1 μm pore-sized ceramic membrane, and experimental research was performed on a coal-fired power plant. The influence of longitudinal tube spacing on the water recovery and total heat transfer coefficient of the membrane condenser under the condition of high cooling water temperature (greater than 30 °C) was analyzed. The results show that for three different longitudinal spacings of 8, 5, and 3 cm, the amount of reclaimed water first decreases slowly by increasing the cooling water temperature. When the temperature reaches about 36 °C, the amount of reclaimed water decreases significantly. Among the three longitudinal spacings, the membrane module with a spacing of 8 cm has the best water recovery performance. The maximum water flux and maximum total heat transfer coefficient can reach 22.23 kg/(m2•h) and 1068.2 W/(m2•K). The findings from the pilot-scale investigation provide important insights for optimizing large scale water and heat recovery systems.

Lehimli Plakalı Isı Değiştiricisi Isı Transferinin Sayısal Analizi

In this paper, we investigated the heat transfer characteristics of a brazed plate heat exchanger by using Computational Fluid Dynamics (CFD). Heat exchangers were generally analyzed considering both rating and sizing parameters. We just examined the rating parameters such as flow direction, mass flow rate, and the inlet temperature value of hot water on the thermal performance of a brazed plate heat exchanger, numerically. We also employed experiments for getting comparative results and each experiment was performed under the same conditions selected in the numerical simulations. Theoretical calculations were achieved by using the Logarithmic Mean Temperature Difference (LMTD) method for a single-phase flow and the thermal efficiency of the heat exchanger was evaluated for different conditions. The maximum total heat transfer rate was calculated about 3.1 kW for experimental study whereas this value was lower than 3 kW for numerical study in counter flow conditions. Another important result was that the effectiveness values of the heat exchanger were calculated higher under counter flow conditions as expected. The numerical results were in good agreement with the experimental data used in the study.

Optimization of operating parameters of earth air tunnel heat exchanger for space cooling: Taguchi method approach

In the present study, CFD-based parametric analysis is carried out to optimise the parameters affecting the temperature drop and heat transfer rate achieved from earth air tunnel heat exchanger (EATHE) system. ANSYS FLUENT 15.0 is used for CFD analysis, and k-ε model and energy equation were considered to define the turbulence and heat transfer phenomena. For a straight EATHE system configuration, four design and operating parameters, i.e., diameter of the pipe (A), length of pipe (B), inlet air velocity (C), and inlet air temperature (D), are considered at four different levels in Taguchi method. The Taguchi method is used to obtain maximum air temperature drop and heat transfer rate. The best combination of parameters for achieving a maximum drop in air temperature is A1B4C1D4 and that for obtaining maximum total heat transfer rate is A4B4C4D4. Statistical analysis reveals the percentage contribution of different factors for air temperature drop in the following order: inlet air temperature (57.80%), diameter of pipe (20.66%), length of pipe (12.03%), and air velocity (9.51%), while, for heat transfer rate, pipe diameter (53.28%), inlet air temperature (30.87%), air velocity (9.40%), and length of pipe (6.45%).

Correlations and optimization of a heat exchanger with offset fins by genetic algorithm combining orthogonal design

In this paper, the study is focused on a double flow plate-fin heat exchanger (PFHE) which heat transfer element is staggered offset fin, and heat transfer model and the energy equations for the structure have been established, seven geometric parameters such as the fin height, fin length and fin wrinkling angle are taken as the decision variables for optimization. A genetic algorithm (GA) combined with orthogonal design is used to search for the optimal overall structure and the correlations about the fin heat transfer factor j and the friction factor f. The maximum total heat transfer rate and the minimum total pressure drop are taken as objective functions in the GA, respectively. Performance of the optimized structure is evaluated and correspondingly the heat transfer and hydrodynamic characteristics of the full-size PFHE are calculated by using a porous media approach. Numerical results show that the total heat transfer rate of the optimized structure is improved about 6.2% comparing with the original design, the total pressure drop decreases by about 40% and the volume can reduce about 2.7%.

Multi-objective optimization of a plain fin-and-tube heat exchanger using genetic algorithm

In the present paper, a plate fin-and-tube heat exchanger (PFTHE) is considered for optimization with air and water as working fluid, four geometric variables are taken as parameters for optimization, a Genetic Algorithm (GA) was used to search for the optimal structure sizes of the PFTHE, the maximum total heat transfer rate and the minimum total pressure drop are taken as objective functions in GA, respectively. Performance of the optimized result was evaluated and correspondingly the total heat transfer rate, the total pressure drop, the heat transfer coefficient and the local Nusselt number, j-factor and friction factor ξ are calculated respectively. Results show that the total heat transfer rate of the optimized heat exchanger increased by about 2.1–9.2% comparing with the original one, the heat transfer coefficient increased by about 8.2–14.7% and the total pressure drop decreased by about 4.4–8% in the range of Re = 1200–14000.

Analytical Solutions of Heat Transfer and Film Thickness in Thin-Film Evaporation

A mathematical model predicting heat transfer and film thickness in thin-film region is developed herein. Utilizing dimensionless analysis, analytical solutions have been obtained for heat flux distribution, total heat transfer rate per unit length, location of the maximum heat flux and ratio of conduction thermal resistance to convection thermal resistance in the evaporating film region. These analytical solutions show that the maximum dimensionless heat flux is constant which is independent of the superheat. Maximum total heat transfer rate is determined for a given film region. The ratio of conduction thermal resistance to convection thermal resistance is a function of dimensionless film thickness. This work will lead to a better understanding of heat transfer and fluid flow occurring in the evaporating film region.

Effects of hydrogen concentration on the emission and heat transfer of a premixed LPG-hydrogen flame

This paper describes an experimental study of the effect of hydrogen concentration on the emission and heat transfer characteristics of a laminar premixed LPG-hydrogen flame. The mole fraction of hydrogen in the fuel mixture was varied from 0% to 50%. The equivalence ratio of the fuel/air mixture was kept at stoichiometry and the mixture jet Reynolds number was fixed at Re = 1500 for most of the tests. The results show that upon varying hydrogen content in the fuel mixture, there is a corresponding change in the appearance, pollutant emissions and heat transfer characteristics of the flame.As the volume flow rate of the fuel/air mixture was increased at higher hydrogen concentration, the height of the inner reaction cone reduced monotonically, indicative of an increase in the laminar burning speed of the mixture. Also, the flame temperature increased, the CO emission decreased and the NOx emission increased. Due to the shortened flame length and enhanced flame temperature, the maximum stagnation point heat flux increased with its position shifting closer to the burner exit at higher hydrogen concentration. Similarly, the maximum total heat transfer rate occurred at smaller nozzle-to-plate distance and the heat transfer was enhanced.

Performance analysis of a membrane-based energy recovery ventilator: Effects of membrane spacing and thickness on the ventilator performance

A mathematical model was built to predict the thermal–hydraulic performance of a membrane-based energy recovery ventilator. Calculations were conducted to investigate the effects of the membrane spacing (channel height) and membrane thickness on the ventilator performance under equal fan power conditions. The results show that for a fixed fan power, as the channel height increases, the total heat transfer rate initially increases, after reaching a maximum at a certain channel height, turns to decrease. A larger fan power leads to a larger total heat transfer rate, with the maximum total heat transfer rate occurring at a smaller channel height. As the channel height or the fan power increases, the enthalpy effectiveness decreases. Further, as the membrane thickness increases, both the total heat transfer rate and enthalpy effectiveness decrease.

OPTIMALLY STAGGERED FINNED CIRCULAR AND ELLIPTIC TUBES IN FORCED CONVECTION

This work presents a three-dimensional (3-D) numerical and experimental geometric optimization study to maximize the total heat transfer rate between a bundle of finned tubes in a given volume and a given external flow both for circular and elliptic arrangements, for general staggered configurations. The optimization procedure started by recognizing the design limited space availability as a fixed volume constraint. The experimental results were obtained for circular and elliptic configurations with a fixed number of tubes (12), starting with an equilateral triangle configuration, which fitted uniformly into the fixed volume with a resulting maximum dimensionless tube-to-tube spacing S/2b = 1.5, where S is the actual spacing and b is the smaller ellipse semi-axis. Several experimental configurations were built by reducing the tube-to-tube spacings, identifying the optimal spacing for maximum heat transfer. Similarly, it was possible to investigate the existence of optima with respect to other two geometric degrees of freedom, i.e., tube eccentricity and fin-to-fin spacing. The results are reported for air as the external fluid in the laminar regime, for 125 and 100 Re 2b , where 2b is the ellipses smaller axis length. Circular and elliptic arrangements with the same flow obstruction cross-sectional area were compared on the basis of maximum total heat transfer. This criterion allows one to quantify the heat transfer gain in the most isolated way possible, by studying arrangements with equivalent total pressure drops independently of the tube cross section shape. This paper reports three-dimensional (3- D) numerical optimization results for finned circular and elliptic tubes arrangements, which are validated by direct comparison with experimental measurements with good agreement. Global optima with respect to tube-to-tube spacing, eccentricity and fin-tofin spacing ( 0.5 e 0.5, S/2b and 06 . 0 f for 125 and 100 Re 2b , respectively) were found and reported in general dimensionless variables. A relative heat transfer gain of up to 19% is observed in the optimal elliptic arrangement, as compared to the optimal circular one. The heat transfer gain, combined with the relative material mass reduction of up to 32% observed in the optimal elliptic arrangement in comparison to the circular one, show the elliptical arrangement has the potential for a considerably better overall performance and lower cost than the traditional circular geometry.

Optimally staggered finned circular and elliptic tubes in forced convection

This work presents a numerical and experimental geometric optimization study to maximize the total heat transfer rate between a bundle of finned or non-finned tubes in a given volume and a given external flow both for circular and elliptic arrangements, for general staggered configurations. The optimization procedure started by recognizing the design limited space availability as a fixed volume constraint. The experimental results were obtained for circular and elliptic configurations with a fixed number of tubes (12), starting with an equilateral triangle configuration, which fitted uniformly into the fixed volume with a resulting maximum dimensionless tube-to-tube spacing S/2 b=1.5, where S is the actual spacing and b is the smaller ellipse semi-axis. Several experimental configurations were built by reducing the tube-to-tube spacings, identifying the optimal spacing for maximum heat transfer. Similarly, it was possible to investigate the existence of optima with respect to other two geometric degrees of freedom, i.e., tube eccentricity and fin-to-fin spacing. The results are reported for air as the external fluid, in the range 852⩽ Re L⩽8520, where L is the swept length of the fixed volume. Circular and elliptic arrangements with the same flow obstruction cross-sectional area were compared on the basis of maximum total heat transfer. This criterion allows one to quantify the heat transfer gain in the most isolated way possible, by studying arrangements with equivalent total pressure drops independently of the tube cross-section shape. The first part of the paper reports two-dimensional numerical optimization results for non-finned circular and elliptic tubes arrangements, which are validated by direct comparison with experimental measurements with good agreement. The second part of the paper presents experimental optimization results for non-finned and finned circular and elliptic tubes arrangements. A relative heat transfer gain of up to 20% is observed in the optimal elliptic arrangement, as compared to the optimal circular one. Both local optimal eccentricity ( S/2 b=0.25 and fixed fin-to-fin spacing) and local optimal fin-to-fin spacing (circular tube and S/2 b=0.5) are shown to exist. Such findings motivate the search for global optima with respect to tube-to-tube spacing, eccentricity and fin-to-fin spacing in future three-dimensional numerical optimization studies.

Optimization study and heat transfer comparison of staggered circular and elliptic tubes in forced convection

In this study, a two-dimensional (2-D) heat transfer analysis was performed in circular and elliptic tube heat exchangers. The finite element method was used to discretize the fluid flow and heat transfer governing equations and a 2-D isoparametric, four-noded, linear element was implemented for the finite element analysis program, FEAP (O.C. Zienkiewicz, R.L. Taylor, The Finite Element Method, vol. 1, McGraw-Hill, London, 1989, Chapter 15). The numerical results for the equilateral triangle staggering configuration, obtained with the new element were then validated qualitatively by means of direct comparison to previously published experimental results for circular tubes heat exchangers (G. Stanescu, A.J. Fowler, A. Bejan, Int. J. Heat Mass Transfer 39 (2) (1996) 311–317). Next, a numerical geometric optimization was conducted to maximize the total heat transfer rate between the given volume and the given external flow both for circular and elliptic arrangements, for general staggering configurations. The results are reported for air in the range 300⩽ Re L⩽800, where L is the swept length of the fixed volume. Circular and elliptical arrangements with the same flow obstruction cross-sectional area were compared on the basis of maximum total heat transfer. The effect of ellipses eccentricity was also investigated. A relative heat transfer gain of up to 13% is observed in the optimal elliptical arrangement, as compared to the optimal circular one. The heat transfer gain, combined with the relative pressure drop reduction of up to 25% observed in previous studies (H. Brauer, Chem. Process Eng., August (1964) 451–460; S.N. Bordalo, F.E.M. Saboya, Determinação experimental dos coeficientes de perda de carga em trocadores de calor de tubos circulares e elípticos aletados, in: Proceedings of the 13th COBEM, Congresso Brasileiro de Engenharia Mec a ̂ nica (in Portuguese), Belo Horizonte, Brasil, 1995) show that the elliptical arrangement has the potential for a considerably better overall performance than the traditional circular one.

Experiments on Chimney-Enhanced Free Convection

Experimental results are presented for thermal systems consisting of a vertical, parallel-plate, isothermal heat sink and a chimney. A ridge of maximum total heat transfer is observed with respect to the plate spacing and heat-sink height. The magnitude of heat transfer and the location of optima confirm prior theoretical predictions by Fisher et al (1997) to within 11 percent. Averaged velocity measurements closely match theoretical predictions of mean velocity. Cold inflow at the chimney exit is quantified and shown to decrease overall heat transfer by approximately four percent.

Analysis and optimization of a natural draft heat sink system

An analytical solution for a system consisting of a vertical, parallel-plate, isothermal heat sink, and a chimney is presented. The result is applied to problems in which the size of the overall system is constrained. A ridge of maximum total heat transfer is observed with respect to the plate spacing and heat-sink height. The ridge suggests that heat-sink height may be reduced without diminishing the thermal performance for natural draft cooling. The flow characteristics of optimal systems reveal that systems with and without chimneys dissipate maximum heat when the velocity profiles within the heat sink are fully developed and developing, respectively.

The optimal spacing of parallel plates cooled by forced convection

This paper reports the optimal board-to-board spacing and maximum total heat transfer rate from a stack of parallel boards cooled by laminar forced convection. The optimal spacing is proportional to the board length raised to the power 1/2, the property group (μα) 1 4 , and (ΔP) − 1 4 , where ΔP is the pressure head maintained across the stack. The maximum total heat transfer rate is proportional to (ΔP) 1 2 , the total thickness of the stack ( H), and the maximum allowable temperature difference between the board and the coolant inlet. Board surfaces with uniform temperature and uniform heat flux are considered. It is shown that the surface thermal condition (uniform temperature vs uniform heat flux) has a minor effect on the optimal spacing and the maximum total heat transfer rate.

Annual and interannual variability of the North Pacific ocean‐to‐atmosphere total heat transfer

Using the sum of latent and sensible heat fluxes and back radiation between the ocean and the atmosphere to express the total heat transfer, Q(T), the annual and interannual variations of QT over the North Pacific were calculated and analysed. The total heat transfer is positive (atmosphere gains heat from the ocean) for all seasons and all regions of the North Pacific and there is a close relationship with the main ocean currents. The area of maximum heat transfer is the Kuroshio area with an annual variation mainly determined by the Asian monsoon circulation. The maximum heat transfer is in January and the minimum, in June. Conversely, the maximum total heat transfer to the air over the Gulf of Alaska occurs in October. This follows from the interaction of the annual cycle of the oceanic thermal content (which is a maximum in late summer) and the occurrence of stormy conditions in the late fall. The QT over the California Current area is about 70% of that over the Kuroshio at the same latitude on the other side of the basin. The spectrum of the anomalies from the 30-year mean annual cycle show low frequency variations of period about 5 years in the Kuroshio area and about 2 years in the Alaska and California areas. After filtering out the monthly fluctuations, it was found that the Kuroshio area heat transfer was correlated with variations across the central North Pacific to about 170 degrees W. The remainder of the basin, including the area of the North Equatorial Current, was correlated with heating variations in both the California and Alaska Current regions.