The optimal fin spacing for three-row flat tube bank fin mounted with vortex generators

To reduce the size and the weight of heat exchangers, vortex generators (VGs) were punched on fin surface to improve the fin heat transfer performance. This paper is focused on the optimal fin spacing for three-row flat tube bank fin mounted with VGs. The results show, for commonly used fin materials and fin thickness, the optimal fin spacing is about 2 mm in industrial application for the configuration of tube bank fin studied.     

Comparison of heat transfer performance of tube bank fin with mounted vortex generators to tube bank fin with punched vortex generators

The results obtained from naphthalene sublimation heat/mass analogy experiments in selecting the optimum geometrical parameters of tube bank fin heat exchanger with fins mounted with vortex generators are compared with the results obtained from the condensing experiments of the real heat exchangers with vortex generators punched out on the fins. The results declare that VGs pouched or mounted on fin surfaces have only limited effects on heat transfer performance in the studied configurations; naphthalene sublimation method can be used to select fin patterns with reasonable reliability.     

Numerical study of heat transfer enhancement of finned flat tube bank fin with vortex generators mounted on both surfaces of the fin

Tube bank fin heat exchanger is one of the most compact heat exchangers, and it is widely used in industry equipments. The flat tube bank fin heat exchangers with vortex generators (VGs) have significant good heat transfer performance, and are used as radiators of locomotive. Here, we study heat transfer enhancement of a new fin where VGs are mounted on both surfaces of the fin. The heat transfer performance of this pattern is evaluated by a numerical method, and the results are compared with those obtained, under identical mass flow rate, when the VGs are mounted only on one surface of the fin. The results reveal that using this new pattern the height of VGs can be reduced and still obtain satisfactory heat transfer enhancement, while the pressure drop is reduced. The results also reveal that if VGs on one surface of the fin is determined, the locations where VGs are mounted on other surface of the same fin are very important, with configurations studied in this paper, depending on the value of Reynolds number, there exists an optimum location with which best heat transfer performance can be obtained.     

Thermo-hydraulic characterization of a louvered fin and flat tube heat exchanger

In the present study, a whole heat exchanger with a hydraulic diameter of 2.3 mm is tested, which is a minichannel heat exchanger according to the Kandlikar classification. This is a louvered fin and flat tube heat exchanger currently used in car cooling systems, also known as radiator. A glycol-water mixture (60/40 in volume) circulates through the tubes at flows ranging from 100 to 7800 l/h and at a supply temperature of 90 °C. This fluid is cooled with ambient air at a temperature of 20 °C and at frontal air velocities varying between 0.5 and 7 m/s. The thermohydraulic performance of the heat exchanger is compared with the classical correlations given in the literature for the heat transfer and the friction factor calculation. On the glycol-water side the heat exchanger is characterized for Reynolds numbers from 30 to 8000. A first comparison is carried out with the correlations available in the literature with a purely predictive model by obtaining a predictive value with a systematic under prediction lower than 10%. In a second step a semi-empirical model is considered to identify the experimental heat transfer coefficients for this application.     

The effects of span position of winglet vortex generator on local heat/mass transfer over a three-row flat tube bank fin

The naphthalene sublimation method was used to study the effects of span position of vortex generators (VGs) on local heat transfer on three-row flat tube bank fin. A dimensionless factor of the larger the better characteristics, JF, is used to screen the optimum span position of VGs. In order to get JF, the local heat transfer coefficient obtained in experiments and numerical method are used to obtain the heat transferred from the fin. A new parameter, named as staggered ratio, is introduced to consider the interactions of vortices generated by partial or full periodically staggered arrangement of VGs. The present results reveal that: VGs should be mounted as near as possible to the tube wall; the vortices generated by the upstream VGs converge at wake region of flat tube; the interactions of vortices with counter rotating direction do not effect Nusselt number (Nu) greatly on fin surface mounted with VGs, but reduce Nu greatly on the other fin surface; the real staggered ratio should include the effect of flow convergence; with increasing real staggered ratio, these interactions are intensified, and heat transfer performance decreases; for average Nu and friction factor (f), the effects of interactions of vortices are not significant, f has slightly smaller value when real staggered ratio is about 0.6 than that when VGs are in no staggered arrangement. A cross section area of flow passage [m²] - A _mim minimum cross section area of flow passage [m²] - a width of flat tube [m] - b length of flat tube [m] - B _pT lateral pitch of flat tube: B _pT = S ₁/T _p - d _h hydraulic diameter of flow channel [m] - D _naph diffusion of naphthalene [m²/s] - f friction factor: f = pd _h/(Lu ² _max/2) - h mass transfer coefficient [m/s] - H height of winglet type vortex generators [m] - j Colburn factor [–] - JF a dimensionless ratio, defined in Eq. (23) [–] - L streamwise length of fin [m] - L _PVG longitudinal pitch of vortex generators divided by fin spacing: L _pVG = l _VG/T _p - l _VG pitch of in-line vortex generators [m] - m mass [kg] - m mass sublimation rate of naphthalene [kg/m²·s] - Nu Nusselt number: Nu = d _h/ - P pressure of naphthalene vapor [Pa] - p non-dimensional pitch of in-line vortex generators: p = l _VG/S ₂ - Pr Prandtl number [–] - Q heat transfer rate [W] - R universal gas constant [m²/s²·K] - Re Reynolds number: Re = ·u _max·d _h/ - S ₁ transversal pitch between flat tubes [m] - S ₂ longitudinal pitch between flat tubes [m] - Sc Schmidt number [–] - Sh Sherwood number [–]: Sh = hd _h/D _naph - Sr staggered ratio [–]: Sr = (2Hsin – C)/(2Hsin) - T _p fin spacing [m] - T temperature [K] - u _max maximum velocity [m/s] - u average velocity of air [m/s] - V volume flow rate of air [m³/s] - x,y,z coordinates [m] - z sublimation depth[m] - heat transfer coefficient [W/m²·K] - heat conductivity [W/m·K] - viscosity [kg/m²·s] - density [kg/m³] - attack angle of vortex generator [°] - time interval for naphthalene sublimation [s] - fin thickness, distance between two VGs around the tube [m] - small interval - C distance between the stream direction centerlines of VGs - p pressure drop [Pa] - 0 without VG enhancement - 1, 2, I, II fin surface I, fin surface II, respectively - atm atmosphere - f fluid - fin fin - local local value - m average - naph naphthalene - n,b naphthalene at bulk flow - n,w naphthalene at wall - VG with VG enhancement - w wall or fin surface     

Computation of heat transfer enhancement in a plate‐fin heat exchanger with triangular inserts and delta wing vortex generator

Longitudinal vortices disrupt the growth of the thermal boundary layer, thereby the vortex generators producing the longitudinal vortices are well known for the enhancement of heat transfer in compact heat exchangers. The present investigation determines the heat transfer characteristics with secondary flow analysis in plate fin triangular ducts with delta wing vortex generators. This geometrical configuration is investigated for various angles of attack of the wing i.e. 15°, 20°, 26° and 37° and Reynolds numbers 100 and 200. The constant wall temperature boundary condition is used. The solution of the complete Navier Stokes equation and the energy equation is carried out using the staggered grid arrangement. The performance of the combination of triangular secondary fins and delta wing with stamping on slant surfaces has also been studied. Copyright © 2009 John Wiley & Sons, Ltd.     

Quantitative infrared investigation of local heat transfer in a circular finned tube heat exchanger assembly

This work deals with the local heat transfer coefficient evaluation over the fin of the second row of a staggered circular finned tube heat exchanger assembly. The coefficient distribution is determined by using a transient technique and by calculating the energy balance during the fin cooling. The calculation model takes into account radiation with the surrounding and lateral heat conduction into the material. The method uses infrared measurements and integration between time bounds that depend on space. It is proposed to choose the integration bounds with an original criterion based on local heat transfer. Validation is performed on the reference case consisting in a thin plate in an aerodynamically and thermally developing channel flow. Then, distributions of Nusselt number on the circular fin are presented for several Reynolds numbers. The high resolution of the whole method and set-up allow detecting thermal imprints of developing horseshoe vortices. These imprints are analyzed by following their angular evolution around the tube.     

Wing-type vortex generators for fin-and-tube heat exchangers

The effect of wing-type vortex generators on heat transfer and pressure drop of a fin-and-tube heat exchanger element was investigated. Local heat transfer was measured by liquid crystal thermography on the fin in the Reynolds number range of 600–2700. Flow losses were estimated from the measured pressure drop of an element. Delta winglets were used as vortex generators. Four fin-and-tube configurations were tested, an inline and a staggered arrangement, each with plain fins and with fins with a pair of vortex generators behind each tube. For the inline tube arrangement the vortex generators increase the heat transfer by 55–65% with a corresponding increase of 20–45% in the apparent friction factor. Results indicate that the vortex generators have the potential to reduce considerably the size and mass of heat exchangers for a given heat load.     

Metal foam and fin implementation into a triple concentric tube heat exchanger over melting evolution

It is believed that it is going to be a sizeable mismatch between supply and demand when it comes to renewable resources. Lately, researchers are on course to compensate for the unpredictabilityof such resources by the employment of phase change materials (PCMs). Having multiple advantages, PCMs generally suffer from inadequate thermal conductivity which causes prolonged transition procedures. To tackle this issue, this study is fixated on two parameterswhich are linked to fins addition and porous media incorporation in a melting process within a triple concentric tube heat exchanger (TCTHX). The results provided by multiple cases underlined the significance of natural convection in the bare system, although finned and copper-metal-foam cases outshine buoyancy forces by roughly 45% and 97%, respectively. Material is a major determent when it comes to the selection of porous media as Al₂O₃ registered the weakest performance among SiC, Ni and Cu, however, it managed to speed up the process by 75% which still is much higher than the finned system, implying that porous media is of higher priority over fins. The best scenario transpiredwhile fins and copper metal foam were integrated as 26% and 97% soars in efficacy have been obtained compared to individual incorporation of porous media and fins, respectively.     

Sizing problem for a cross flow heat exchanger with wing-type vortex generator

In the present study, sizing of a single pass cross flow heat exchanger with unmixed fluid streams has been investigated. The heat exchanger is a cross flow heat exchanger. It has overall dimensions of 20 × 20 × 20 cm. Two the most common heat exchanger design problems are the rating and sizing problem. Sizing problems deal with designing an exchanger and determining its physical size to meet the specified heat duty, pressure drops and other considerations. It means the determination of the exchanger construction type, flow arrangement, heat transfer surface geometries and materials, and the physical sizes of an exchanger to meet specified heat transfer and pressure drop. In this study, the physical size (length, width, height, mass flow rates of both fluids and surface areas on each side of the exchanger) are determined. Inputs to the sizing problem are surface geometries, fluid mass flow rates, inlet and outlet fluid temperatures and pressure drop on each side. Dimensions of L _a , L _b , and L _c for the selected surfaces were investigated such that the design meets the heat duty and pressure drops on both sides exactly.     

Laminar heat transfer characteristics of internally finned tube with sinusoidal wavy fin

Comparative numerical study of laminar heat transfer characteristics of annular tubes with sinusoidal wavy fins has been conducted both experimentally and numerically with Re = 299–1,475. The uniform heat flux is imposed on the tube outside wall surface. Two tube materials (copper and stainless steel) are considered. It is found that the fluid temperature profile is not linear but convex along the flow direction due to the axial heat conduction in tube wall, and the effects of axial heat conduction on the heat transfer decreases with an increase in Reynolds number or decrease in tube wall thermal conductivity. The axial distributions of local Nusselt number could reach periodically fully developed after 3–5 cycles. The convectional data reduction method based on the traditional method should be improved for tube with high thermal conductivity or low Reynolds numbers, Otherwise, the heat transfer performance of internally finned tube may be underestimated.     

Numerical simulation of flow in a circular duct fitted with air‐jet vortex generators

Most of the fundamental studies of the use of air‐jet vortex generators (AJVGs) have concentrated on their potential ability to inhibit boundary layer separation on aerofoils. However, AJVGs may be of use in controlling or enhancing certain features of internal duct flows. For example, they may be of use in controlling the boundary layer at the entrance to engine air intakes, or as a means of increasing mixing and heat transfer. The objective of this paper is to analyse the flow field in the proximity of an air‐jet vortex generator array in a duct by using two local numerical models, i.e. a simple flat plate model and a more geometrically faithful sector model. The sector model mirrors the circular nature of the duct's cross‐section and the centre line conditions on the upper boundary. The flow was assumed fully turbulent and was solved using the finite volume, Navier–Stokes Code CFX 4 (CFDS, AEA Technology, Harwell) on a non‐orthogonal, body‐fitted, grid using the k–ε turbulence model and standard wall functions. Streamwise, vertical and cross‐stream velocity profiles, circulation and peak vorticity decay, peak vorticity paths in cross‐stream and streamwise direction, cross‐stream vorticity profiles and cross‐stream wall shear stress distributions were predicted. Negligible difference in results was observed between the flat plate and the sector model, since the produced vortices were small relative to the duct diameter and close to the surface. The flow field was most enhanced, i.e. maximum thinning of the boundary layer, with a configuration of 30° pitch and 75° skew angle. No significant difference in results could be observed between co‐ and counter‐rotating vortex arrays. Copyright © 2002 John Wiley & Sons, Ltd.     

Velocity field and heat transfer in a vortex flow exchanger

This study deals with the flow of newtonian and non-newtonian fluids, inside a cylindrical flat cavity, provided with a tangential injection device, model of heat exchangers with spiral flow.The dynamic analysis which requires essentially a laser Doppler velocimeter focusses on the determination of the velocity field and brings out the existence of secondary flows which are important for heat transfer.In turbulent regime, the law of exchange agrees with the Chilton-Colburn analogy. On the other hand, for laminar flow, it becomes necessary to include the variations with temperature of the consistency (K); the Nusselt number (Nu) not only depends on the flow rate, but also on the density of transferred heat flux.

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Geschwindigkeitsfeld und Wärmeübertragung in einem Wärmeüberträger mit spiralförmiger Strömung

Zusammenfassung Diese Studie betrifft die Strömung von Newton'schen oder Nicht-Newton'schen Flüssigkeiten innerhalb eines flachen zylindrischen Hohlraumes, der mit einem tangentialen Einspritzungssystem versehen ist und als Modell für Wärmeaustauscher mit spiralförmiger Strömung dient.Die dynamische Analyse, die das Laser-Dopplerverfahren (Laser-Velocimetrie) erfordert, kennzeichnet das Geschwindigkeitsfeld und beweist die Existenz von Sekundärströmungen, die für die Wärmeübertragung wichtig sind.Für den turbulenten Betrieb folgt das Gesetz des Wärmeaustausches der Chilton-Colburn Analogie. Was den laminaren Betrieb angeht, wird es erforderlich, die Variationen der BeschaffenheitK mit der Temperatur zu berücksichtigen. Die Nusseltsche Zahl hängt natürlich vom dynamischen Betrieb ebenso wie von der übertragenen Leistung ab.

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Nomenclature Nu=[/S T]D _h/ Nusselt number - Re=V ₀ D _h/ Reynolds number - Re _g=V ₀ ^2–n D _n ^h /K generalized Reynolds number - Pr=C _p / Prandtl number - Pr _g=C _p[V ₀/D _h] ^n–1 K/ generalized Prandtl number - H height of cylinders (m) - H dimension of the entrance device (m) - S ₁ tested section - S ₂ tested section - shear stress (Pa) - K consistency (Pa s ⁿ ) - shear rate (s^–1) - n power law index - a, b constants in the consistency formulaK=a exp [–b t] - a, b constants in the rheological indexn=a exp [b T] - T, T _e,T _p temperature, inlet temp., wall temp. - p pressure (Pa) - p pressure drop (Pa) - V velocity vector (components:V ₁,V ₂,V ₃) (m/s) - V ₀ mean velocity (m/s) - X ₁,X ₂,X ₃ cylindrical coordinate system - C _f friction factor - R ₁,R ₂ radius (m) - heat flux (W) - thermal conductivity (W/(m °C)) - C _p specific heat of fluid (J/(kg C°)) - dynamic viscosity (Pa s) - Q _v volumic flow rate (m³/s) - =/S heat flux density (W/m²) - S exchange area (m²)     

Calculation of overall heat transfer coefficients in a triple tube heat exchanger

In previous studies, calculation of overall heat transfer coefficients in a triple tube heat exchanger (TTHE) involved assumptions or approaches those are not valid in all cases. In this study a more generic way of calculating overall heat transfer coefficients in a TTHE has been developed. Consequently, temperature profiles of all streams in a TTHE in the axial direction were determined. An effective overall heat transfer coefficient that is related to the total resistance to heat transfer in the TTHE, was also determined to facilitate comparison of a TTHE to an equivalent double tube heat exchanger.     

Flow and heat transfer characteristics behind vortex generators - A benchmark dataset

The velocity field and wall heat transfer distributions for internal flows in the presence of longitudinal vortices have been experimentally investigated. A transient method based on temperature measurement with thermochromic liquid crystals was used to obtain the heat transfer distribution behind a tetrahedral, full-body vortex generator. With the focus on the longitudinal vortices, the flow field was captured by a three-component particle image velocimetry system. Mean values as well as velocity fluctuations have been assessed. The combined investigation of heat transfer and flow field describes in detail the physical conditions. For a channel Reynolds number of 300,000 a dataset has been obtained, which can be used for validation of numerical models.