An analytical prediction for performance and optimization of an annular fin assembly of trapezoidal profile under dehumidifying conditions

The present study demonstrates an analysis analytically to determine the performances and optimum design of wet annular fin assemblies of the trapezoidal profile. The Frobenius power series method is adopted as an analytical tool to solve the governing differential equation of the above type of wet fin assemblies. The performance parameters, namely, the surface efficiency and augmentation factor are determined. The present model has also ability to predict the performances of a wet fin assembly of triangular fin geometry for the selection of the very small value of the tip thickness. As the present study establishes an analytical model, it can be easily employed in determination of an optimum design condition. Both the performance and optimization study have been made by two approaches of the analysis described based on the handling of the psychrometric properties at the tip as a function of temperatures. Finally, it can be highlighted that the performances and optimum conditions of a wet fin assembly are not only dependent upon the psychrometric properties of air but also dependent upon the approach selected for calculating the energy transferred by the mechanism of mass transfer.     

Performance of the herringbone wavy fin under dehumidifying conditions

An experimental study reporting the airside performance of the herringbone wavy fin geometry in wet conditions is conducted. In the visualization of the condensate flow pattern, a very special “locally dry” spot of the corrugation wavy channel having a corrugation angle of 15° and a fin spacing of 8.4 mm is seen. This phenomenon is related to the recirculation of the airflow across the apex. Conversely, this phenomenon is not so clearly seen either for a fin pitch of 2.6 mm with a corrugation angle of 15° or a corrugation angle of 25°. Flow visualization of the non-uniform distribution of the condensation in the facets results in a dependence between axial length and friction factor. Based on the present test results, airside performance in terms of Nusselt number and Fanning friction factor for the present herringbone wavy fin geometry in wet conditions are developed. The mean deviations of the proposed correlations are 2.52% and 4.81%, respectively.     

Performance analysis and optimization of elliptic fins circumscribing a circular tube

The performance of elliptic disc fins has been analyzed using a semi-analytical technique. It has been shown that the efficiency of such fins can also be predicted very closely using the sector method. However, the equivalent annulus method is not suitable for this fin geometry. A method for the optimum design of fins, using a constraint of either fin volume or rate of heat dissipation has also been suggested. Optimum elliptical fins dissipate heat at a higher rate compared to an annular fin when space restriction exists on both sides of the fin. Even when the restriction is on one side only, the performance of elliptical fin is comparable to that of eccentric annular fin for a wide parametric range.     

Effect of fin thickness on the air-side performance of wavy fin-and-tube heat exchangers under dehumidifying conditions

This study investigated the effect of fin thickness on the air-side performance of wavy fin-and-tube heat exchangers under dehumidifying conditions. A total of 10 samples were tested with associated fin thickness (δ_f) of 0.115 mm and 0.25 mm, respectively. For a heat exchanger with two rows (N = 2) and fin pitch F_p of 1.41 mm, the effect of fin thickness on the heat transfer coefficient is more pronounced. The heat transfer coefficients for δ_f = 0.25 mm is about 5–50% higher than those for δ_f = 0.115 mm whereas the pressure drop for δ_f = 0.25 mm is about 5–20% higher. The unexpected difference in heat transfer coefficient subject to fin thickness is attributable to better interactions between the directed main flow and the swirled flow caused by the condensate droplet for δ_f = 0.25 mm. The maximum difference in heat transfer coefficients for N = 2 and F_p = 2.54 mm subject to the influence of fin thickness is reduced to about 20%, and there is no difference in heat transfer coefficient when the frontal velocity is above 3 m/s. For N ⩾ 4 and F_p = 2.54 mm, the influence of fin thickness on the heat transfer coefficients diminishes considerably. This is because of the presence of tube row, and the unsteady/vortex shedding feature at the down stream of wavy channel. Based on the present test results, a correlation is proposed to describe the air-side performance for wavy fin configurations, the mean deviations of the proposed heat transfer and friction correlations are 7.9% and 7.7%, respectively.     

The computed characteristics of turbulent flow and convection in concentric circular annuli. Part III. Alternative thermal boundary conditions

Numerically predicted values of the Nusselt number are presented for annuli with uniform heating of the outer surface only, uniform equal heating of both surfaces, and uniform heating of one surface and equal cooling of the other. Results are also presented for a parallel-plate channel with isothermal heating of one wall and equal isothermal heating of both walls. The predictions are based on semi-theoretical expressions for the dimensionless turbulent shear stress but are insensitive to the limited empiricism within the model for convection. The predicted values of Nu are in good agreement with the somewhat limited experimental data for these less frequently encountered thermal boundary conditions. The results encompass all values of the Prandtl number and all values of the Reynolds number of practical concern. These values, together with those in a preceding paper for uniform heating of the inner wall only, and together with the generalizations described in a succeeding paper, permit the accurate prediction of the Nusselt number from theoretically based algebraic expressions for essentially all of the thermal conditions that result in fully developed convection.     

Simultaneous heat and mass transfer characteristics for wavy fin-and-tube heat exchangers under dehumidifying conditions

The present study proposes a new reduction method to calculate the heat and mass transfer characteristics of the wavy fin-and-tube heat exchangers under dehumidifying conditions. For fully wet conditions, the sensible heat transfer and mass transfer characteristics are relatively insensitive to the inlet relative humidity. The heat and mass transfer performances show appreciable influence of fin spacing at 1-row configuration. Both the heat and mass transfer performances increase when the fin spacing is reduced. However, the difference becomes less noticeable when Re_Dc > 3000. For 1-row configuration, larger wave height shows much larger difference with the fin spacing. However, the effect of inlet conditions and geometrical parameters on the heat and mass performance becomes less significant with the rise of number of tube rows. Test results show that the heat and mass transfer analogy is roughly applicable (the ratios of h_c,o/h_d,oC_p,a are in the range 0.6–1.1, and is insensitive to change of fin spacing). The correlations are proposed to describe the heat and mass transfer characteristics. These correlations can describe 94.19% of the j_h factors within 15% and 83.72% of the j_m factors within 15%. Correspondingly, 93.02% of the ratios of h_c,o/h_d,oC_p,a are predicted by the proposed correlation within 15%.     

Improvement in performance on laminar counterflow concentric circular heat exchangers with external refluxes

Considerable improvement of heat transfer in laminar counterflow concentric heat exchangers is obtainable by inserting in parallel an impermeable, resistless sheet to divide an open duct into two subchannels for double-pass operations with external refluxes. Efficiency improvement in heat transfer has been investigated analytically by using an orthogonal expansion technique. The results of improvement in heat transfer efficiency are represented graphically and compared with those in a single-pass operation. The influences of improvement-sheet location and reflux ratio on the enhancement of transfer efficiency as well as on the increment of power consumption have been discussed.     

Heat-transfer efficiency improvement of double-pass concentric circular heat exchangers under uniform wall fluxes

The new design of the double-pass operation with external recycle is proposed to apply to enhance the heat-transfer efficiency of concentric circular heat exchangers under uniform wall fluxes. The mathematical formulation was developed theoretically and the analytical solutions were achieved by using the orthogonal expansion technique with the eigenfunctions expanding in terms of an extended power series. The theoretical predictions were represented graphically and compared with those obtained from the single-pass device without external recycle and the previous work [C.D. Ho, S.C. Yeh, Improvement in device performance on laminar counterflow concentric circular heat exchangers with uniform wall flux, International Journal of Heat and Mass Transfer 49 (2006) 202–2032] under the different flow pattern. The results show that a considerable heat-transfer efficiency improvement was obtained with a suitable adjustment of the subchannel thickness ratio and recycle ratio. The power consumption increment due to the device with external recycle was also discussed in this study.     

内翅式套管相变蓄热器性能研究

相变蓄热能有效地解决太阳能利用过程中供需不匹配的问题。文章对内翅式太阳能套管式相变蓄热器的蓄热性能进行了研究,结果表明:供热流体入口速度越大,熔化过程所需要的时间越短;速度为2.5 m/s时,熔化过程需要225 min左右;与未加翅片的蓄热器相比,在相变材料侧增加环形翅片时蓄热时间缩短10%左右。     

A review on reduction method for heat and mass transfer characteristics of fin-and-tube heat exchangers under dehumidifying conditions

This paper presents a review of data reduction method for heat and mass transfer characteristics of fin-and-tube heat exchangers with dehumidification. There are many reduction methods for fin-and-tube heat exchangers under dehumidifying conditions. The data reduction methods being reviewed includes the original Threlkeld method, direct method, the equivalent dry-bulb method, tube by tube method, the fully wet and fully dry tiny circular fin method, and the finite circular fin method. Among these methods, the original Threlkeld method, direct method, the equivalent dry-bulb method are lumped method while others can divide the fin-and-tube heat exchangers into small segments for more accuracy in handling the surfaces to be fully dry, fully wet, or partially wet. In addition, the mass transfer characteristics can be obtained from the modified process line equation incorporated with the preceding methods. It should be noted that the conventional assumption of constant ratio (h_c,o/h_d,oC_p,a ≈ constant) is actually incorrect. This present paper can be used as the first guideline for the researcher for reducing the experimental data for fin-and-tube heat exchangers under dehumidifying conditions.     

A combined approach for determining the thermal performance of radiant barriers under field conditions

This paper deals with a combined approach for assessing the thermal performance of radiant barriers under field conditions, based both on dynamic simulations and field measurements. The methodology involves the combination of model predictions and experimental results of a complex roof including a radiant barrier installed on a dedicated test cell. During the empirical validation of the building thermal model and more particularly thanks to the results of sensitivity analysis, simplifications of the model were made. These considerations lead to successive simplified versions of the model and finally a very simplified one, which is used to determine the thermal resistance of the complex roof. We first present the detailed thermal model, elaborated with a prototype of building simulation code. We then describe the experimental test cell and put the emphasis on the details of the roof. The simplification of the detailed model is then explained and the results presented. A value of the thermal resistance is finally obtained and confirms the potential of radiant barriers for a tropical climate.     

Analysis and optimization of convective trapezoidal profile pin fins with internal heat generation

An analysis of trapezoidal profile convective pin fins, with internal heat generation density is presented. The solution of the optimal problem is also given, when either the desired heat dissipation rate or the volume of the pin is specified. The results are presented graphically and in polynomial forms that are particularly useful for computerized calculations. The effect of the fin's profile and thermal conductivity upon the optimum dimensions is discussed. An example serves to demonstrate the usefulness of the method.     

Improvement in device performance on laminar counterflow concentric circular heat exchangers with uniform wall fluxes

The effects of recycle at the ends on the heat transfer through a concentric circular tube with uniform wall fluxes are studied analytically by an orthogonal expansion technique with eigenfunction power series expansion. The heat transfer problem is solved for fully developed laminar velocity profiles in a double-pass circular heat exchanger with ignoring axial conduction and fluid properties of temperature independence. Analytical results show the external recycle can enhance the heat transfer efficiency compared with that in an open tube (without an impermeable circular tube inserted and without recycle). The compensation between the forced-convection increment and heat-transfer driving force decrement are used to study the heat transfer behavior. The effects of the impermeable-tube location on heat transfer efficiency enhancement as well as the power consumption increment have been also discussed.     

Improvement of thermal performance of spiral heat exchanger on hydrogen storage by adding copper fins

The reaction time of hydrogen in metal-hydride vessels (MHVs for short) is strongly influenced by the heat transfer from/to the hydride bed. In the present work an experimental study of the geometric and the operating parameters of a finned spiral heat exchanger has been carried out to identify their influence on the performance of the charging process of the MHV. The experimental results show that the charge time of the reactor is considerably reduced, when finned spiral heat exchanger is used. In addition, the effect of different parameters (flow mass and temperature of the cooling fluid, applied pressure of hydrogen in the case of absorption and desorption) has been discussed and obtained results show that a good choice of these parameters is important.     

Thermal optimization of plate-fin heat sinks with fins of variable thickness under natural convection

In this study, thermal performance of a vertical plate-fin heat sink under natural convection was optimized for the case in which the fin thickness varied in the direction normal to the fluid flow. For this optimization, the averaging approach presented in an earlier paper for the case of the heat sinks under forced convection was extended to study the performance of heat sinks under natural convection. In the case of an air-cooled heat sink, the thermal resistance decreases by up to 10% when the fin thickness is allowed to increase in the direction normal to the fluid flow. However, the difference between the thermal resistances of heat sinks with uniform thickness and the heat sinks with variable thickness decreases as the height decreases and as the heat flux decreases.     

Analysis of two‐dimensional porous fins with variable thermal conductivity

Analysis of porous fins for their higher heat transfer in comparison with solid fins with identical volumes has attracted significant attention. In this paper, a two‐dimensional thermal analysis of a porous fin having variable thermal conductivity coefficient is performed using finite difference method. Heat transfer through porous media is simulated using passage velocity from Darcy's model. The thermal conductivity of the solid phase is considered as a linear function of temperature. It is found that the temperature profile of the fin is completely two‐dimensional even for high Rayleigh and Darcy numbers (Ra = 10³～10⁴, Da = 0.01), because the temperature changes significantly along the transverse axis especially for lower Rayleigh and Darcy numbers. Also, the effects of important nondimensional parameters such as Rayleigh and Darcy numbers, porosity, Nusselt, thermal conductivity, and aspect ratio on the temperature profile are investigated. The results demonstrate that the temperature distribution is strongly dependent on the Rayleigh and Darcy numbers.     

Theoretical and operational thermal performance of a ‘wet’ crystalline silicon PV module under Jamaican conditions

This paper presents the results of the impact of a gravity-fed cooling technique applied to a photovoltaic module. The experiment shows that the technique increases the power output of the module by reversing the negative effects of elevated cell temperature on open circuit voltage, and this without the use of a circulating pump. The cooling technique employs the hydraulic head of water from an upstream source as the driving force that passes water over the back of the module, and this keeps the module temperature constant. The experimental results and the results of mathematical model on which it is predicated on are in very close agreement.     

Wind turbine performance under icing conditions

The wind energy market is in full growth in Quebec but technical difficulties due to cold climate conditions have occurred for most of the existing projects. Thus, icing simulations were carried out on a 0.2 m NACA 63 415 blade profile in the refrigerated wind tunnel of the Anti‐icing Materials International Laboratory (AMIL). The shapes and masses of the ice deposits were measured, as well as the lift and drag forces of the iced profiles. Scaling was carried out based on the 1.8 MW–Vestas V80 wind turbine technical data, for three different radial positions and two in‐fog icing conditions measured at the Murdochville wind farm in the Gaspé Peninsula. For both icing events, the mass of ice accumulated on the blade profile increased with an increase in the radial position. In wet regime testing (first icing event), glaze formed mostly near the leading edge and on the pressure side. It also accumulated by run‐off on the trailing edge of the outer half of the blade. In dry‐regime testing (second icing event), rime mostly accreted on the leading edge and formed horns. For both icing events, when glaze or rime accreted on the blade profile, lift decreased and drag increased. A load calculation using the blade element theory shows that drag force on the entire blade becomes too large compared to lift, leading to a negative torque and the stop of the wind turbine. Torque reduction is more significant on the outer third of the blade. Setting up a de‐icing system only on the outer part of the blade would enable significant decrease of heating energy costs. Copyright © 2007 John Wiley & Sons, Ltd.     

Sensible and latent thermal energy storage with constructal fins

Energy storage is one of the most important components of renewable energy systems. Among different methods, thermal energy storage (TES) in forms of sensible or latent has been the subject of many studies in the past decades. The main difficulty in optimal design of storage tanks is associated with low thermal conductivity of the storing (solid or phase change) material. In fact, the distribution of thermal energy from a source to the body of storing material poses a volume to point problem which is the subject of constructal theory. Therefore, the objective of the present paper is to investigate the transient behavior of a rectangular thermal energy storage tank equipped with fin configurations optimized for heat conduction based on constructal theory. Results of numerical simulations reveal that the more complex configurations perform better in sensible TES systems; almost as well as what is expected based on analytical steady state solutions. However, because of the convection currents in the melting process of a PCM tank, the final full charging time of the latent systems are approximately the same.