The Effects of duct height on heat transfer enhancement of a co-rotating type rectangular finned surface in duct

Experiments were performed to investigate the effect of duct height on heat transfer enhancement of a surface affixed with arrays (7 × 7) of short rectangular plate fins of a co-rotating type pattern in the duct. An infrared imaging system is used to measure detailed distributions of the heat transfer at the endwall along with the fin base. An infrared camera of TVS 8000 with 160 × 120 point In–Sb sensor was used to measure the temperature distributions in order to calculate the local heat transfer coefficients of the representative fin regions. Pressure drop and heat transfer experiments were performed for a co-rotating fin pattern varying the duct height from 20?50 mm. The friction factor calculated from the pressure drop shows that comparatively larger friction occurs for the smaller duct cases and the friction factor slowly decreases with increasing Reynolds number. The effect of duct height on the area-averaged heat transfer results show that heat transfer initially increases with duct height and then finally decreases with increasing the duct height. Detailed heat transfer analysis and iso-heat transfer coefficient contour gives a clear picture of heat transfer characteristics of the overall surface. The relative performance graph indicates that a 25 mm duct is the optimum duct height for the highest thermal performance. In addition, a significant thermal enhancement, 2.8?3.8 times the smooth surface, can be achieved at lower Reynolds number with a co-rotating fin pattern in the duct.     

Air-side performance of louver-finned flat aluminum heat exchangers at a low velocity region

The heat transfer and pressure drop characteristics of heat exchangers having louver fins were experimentally investigated. The samples had small fin pitches (1.0–1.4 mm), and experiments were conducted up to a very low frontal air velocity (as low as 0.3 m/s). Below a certain Reynolds number (critical Reynolds number), the fall-off of the heat transfer coefficient curve was observed. The critical Reynolds number was insensitive to the louver angle, and decreased as the louver pitch to fin pitch ratio (L _p /F _p ) decreased. Existing correlations on the critical Reynolds number did not adequately predict the data. The heat transfer coefficient curves crossed over as the Reynolds number decreased. Possible explanation is provided considering the louver pattern between neighboring rows. Different from the heat transfer coefficient, the friction factor did not show the fall-off characteristic. The reason was attributed to the form drag by louvers, which offsets the decreased skin friction at low Reynolds numbers. The friction factor increased as the fin pitch decreased and the louver angle increased. A new correlation predicted 92% of the heat transfer coefficient and 94% of the friction factor within ±10%.     

An experimental investigation on the airside performance of fin-and-tube heat exchangers having sinusoidal wave fins

The heat transfer and friction characteristics of the heat exchangers having sinusoidal wave fins were experimentally investigated. Twenty-nine samples having different waffle heights (1.5 and 2.0 mm), fin pitches (1.3–1.7 mm) and tube rows (1–3) were tested. Focus was given to the effect of waffle configuration (herringbone or sinusoidal) on the heat transfer and friction characteristics. Results show that the sinusoidal wave geometry provides higher heat transfer coefficients and friction factors than the herringbone wave geometry, and the difference increases as the number of row increases. The j/f ratios of the herringbone wave geometry, however, are larger than those of the sinusoidal wave geometry. Compared with the herringbone wave geometry, the sinusoidal wave geometry yielded a weak row effect, which suggests a superior heat transfer performance at the fully developed flow region for the sinusoidal wave geometry. Possible reasoning is provided considering the flow characteristics in wavy channels. Within the present geometric variations, the effect of waffle height on the heat transfer coefficient was not prominent. The effect of fin pitch was also negligible. Existing correlations highly overpredicted both the heat transfer coefficients and friction factors. A new correlation was developed based on the present data.     

Experimental and numerical investigation on air-side performance of fin-and-tube heat exchangers with various fin patterns

Air-side heat transfer and friction characteristics of five kinds of fin-and-tube heat exchangers, with the number of tube rows (N = 12) and the diameter of tubes (D_o = 18 mm), have been experimentally investigated. The test samples consist of five types of fin configurations: crimped spiral fin, plain fin, slit fin, fin with delta-wing longitudinal vortex generators (VGs) and mixed fin with front 6-row vortex-generator fin and rear 6-row slit fin. The heat transfer and friction factor correlations for different types of heat exchangers were obtained with the Reynolds numbers ranging from 4000 to 10000. It was found that crimped spiral fin provides higher heat transfer and pressure drop than the other four fins. The air-side performance of heat exchangers with the above five fins has been evaluated under three sets of criteria and it was shown that the heat exchanger with mixed fin (front vortex-generator fin and rear slit fin) has better performance than that with fin with delta-wing vortex generators, and the slit fin offers best heat transfer performance at high Reynolds numbers. Based on the correlations of numerical data, Genetic Algorithm optimization was carried out, and the optimization results indicated that the increase of VG attack angle or length, or decrease of VG height may enhance the performance of vortex-generator fin. The heat transfer performances for optimized vortex-generator fin and slit fin at hand have been compared with numerical method.     

Heat transfer for laminar flow in internally finned pipes with different fin heights and uniform wall temperature

An analysis is presented for fully developed laminar convective heat transfer in a pipe provided with internal longitudinal fins, and with uniform outside wall temperature. The fins are arranged in two groups of different heights. The governing equations have been solved numerically to obtain the velocity and temperature distributions. The results obtained for different pipe-fins geometries show that the fin heights affect greatly flow and heat transfer characteristics. Reducing the height of one fin group decreases the friction coefficient significantly. At the same time Nusselt number decreases inappreciably so that such reduction is justified. Thus, the use of different fin heights in internally finned pipes enables the enhancement of heat transfer at reasonably low friction coefficient.Nomenclature A_f dimensionless flow area of the finned pipe, Eq. (8) - a_f flow area of the finned pipe - C_p specific heat at constant pressure - f coefficient of friction, Eq. (12) - H₁, H₂ dimensionless fin height h₁/r_o h₂/r_o - h₁, h₂ fin heights - average heat transfer coefficient at solid-fluid interface - KR fin conductance parameter, k_s/k_f - k_f thermal conductivity of fluid - k_s thermal conductivity of fin - l pipe length - mass flow rate - N number of fins - Nu Nusselt number, Eqs. (15) and (16) - P pressure - Q total heat transfer rate at solid fluid interface - Q_f1, Q_f2 heat transfer rate at fin surface - q_w average heat flux at pipe-wall, Q/(2 r_ol) - R dimensionless radial coordinate r/r_o - Re Reynolds Number, Eq. (13) - r radial coordinate - r_o radius of pipe - r₁, r₂ radii of fin tips - T temperature - T_b bulk temperature - U dimensionless velocity, Eq. (2) - U_b dimensionless bulk velocity - u_z axial velocity - z axial coordinate - angle between the flanks of two adjacent fins - half the angle subtended by a fin - angle between the center-lines of two adjacent fins - angular coordinate - dynamic viscosity - density - dimensionless temperature, Eq. (6) - _b dimensionless bulk temperature     

Experimental study of augmented heat transfer and friction in solar air heater with different orientations of W-Rib roughness

Artificial roughness in the form of ribs is a convenient method for enhancing thermal performance of solar air heaters. This paper presents the experimental investigation of heat transfer and friction factor characteristics of a rectangular duct roughened with W-shaped ribs arranged at an inclination with respect to the flow direction on its underside on one broad wall. W ribs have been tested both pointing in downstream W-down and upstream W-up to the flow. The range of parameters for this study has been decided on the basis of practical considerations of the system and operating conditions. The duct has a width to height ratio (W/H) of 8.0, relative roughness pitch (p/e) of 10, relative roughness height (e/D_h) of 0.03375 and angle of attack of flow (α) of 30-75°. The air flow rate corresponds to Reynolds number between 2300-14,000. The heat transfer and friction factor results have been compared with those for smooth duct under similar flow and thermal boundary condition and thermo-hydraulic performance has been investigated. Thermo-hydraulic performance comparison for different angle of attack of flow shows that W-down arrangement with angle of attack of flow as 60° gives best thermo-hydraulic performance. In addition heat transfer and friction factor correlations have been developed.     

Measurements of heat transfer coefficients between turbulent liquid jets and a radially finned heated disk placed at the bottom of an open vertical circular cavity

 Experiments have been performed to assess the impact of an extended surface on the heat transfer enhancement for axisymmetric, turbulent liquid jet impingement on a heated round disk. The disk, with an array of integral radial fins mounted on its surface, is placed at the bottom of an open vertical circular cavity. Hydrodynamic and heat transfer data were obtained for a dielectric fluorocarbon liquid FC-77. For a fixed circular heater of diameter D=22.23 mm, several geometric parameters were tested: the nozzle diameter (4.42≤d≤9.27 mm), the confining wall diameter of the vertical cavity (22.23≤D _c≤30.16 mm), and the nozzle-to-heater spacing (0.5≤S/d≤5.0). The FC-77 flow rates varied from =0.2 to 11.0 l/min producing Reynolds numbers in the wide interval 700≤Re _d ≤44,000. For d=4.42 mm, the heat transfer response to the separation distance S/d was small but increased gradually with increasing nozzle diameter up to d=9.27 mm. The thermal resistance R _th increased with the confining wall diameter D _c and also with the nozzle diameter d. A minimum value of the thermal resistance of R _th,min=0.4 cm² K/W was attained for a combination of d=4.42 mm, D _c=22.23 mm, S/d=1, and =7.5 l/min. Based on a simplified heat transfer model, reasonable agreement was obtained between measured values of the thermal resistance and the R _th-predictions. The total fin effectiveness ɛ_f was shown to increase with increasing nozzle diameter, but was invariant with the flow rate (or the jet exit velocity). More than a three-fold heat transfer enhancement was realized through the addition of the array of integral radial fins on the heated round disk. Received on 30 August 2000 / Published online: 29 November 2001     

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     

Numerical study of the unsteady flow and heat transfer in channels with periodically mounted square bars

 Numerical investigations of unsteady laminar flow and heat transfer in a channel of height H with periodically mounted square bars of height d = 0.2H arranged side by side to the approaching flow have been conducted for different transverse separation distances of the bars. Five cases with transverse separation distance of 0, 0.5, 1, 1.5 and 2d for a Reynolds number of 300 in a channel with a periodicity length of 2H were studied. The unsteady Navier–Stokes equations and the energy equation have been solved by a finite volume code with staggered grids combined with the SIMPLEC algorithm and a fine grid resolution. Due to the arrangement of bars detached from the channel walls the flow is unsteady with vortex shedding from the bars. The amplitude and mean values of the drag coefficients, skin friction coefficients, friction factor and Nusselt numbers have a strong dependence of the transverse separation distance of the bars. Received on 28 January 2000     

Flow and heat transfer in a pipe with a fin attached to inner wall

Enhancement of heat transfer to the fluid can be done by turbulence promoters such as attached fins to the pipe walls. In this study, the flow field and the heat transfer rates were numerically investigated in a pipe with an internally attached fin. Numerical simulations were conducted for four different types of fluids and for different fin heights and locations, and as the Reynolds number was varied, the effects of the fin on Nusselt number and friction factors were investigated. For all the Reynolds numbers considered in this study, the effect of fin location on the heat transfer rate and friction factor was negligible. As the fin height was increased, the mean Nusselt number and the friction factor also increased in the turbulent flow regimes. For low Prandtl number fluids (Pr = 0.011), the main heat transfer mode is conduction, and hence the mean Nusselt number slightly affected the flow rates.     

An experimental investigation of heat transfer and flow friction characteristics of louvered fin surfaces by the modified single blow technique

This paper describes the development of an experimental facility to determine the heat transfer and flow friction characteristics of heat exchange surfaces by the modified single blow technique and the application of this transient technique to evaluate the performance characteristics of louvered fin heat exchangers. The reliability of implementing the modified single blow technique on the developed test facility is borne out by the good agreement in the heat transfer and flow friction data for the parallel plate test core when compared with theoretical and empirical correlations available in the literature. Performance evaluation of two louvered fin surfaces used mainly for cooling of large land and marine based electrical power generator sets is carried out and compared with similar louvered fin surfaces available in the literature. On the basis of dimensionless area and power factors, it was found that the flat fin is slightly superior in overall performance than its corrugated counterpart for low Reynolds numbers. Both surfaces are however inferior in performance when compared with the flat fin surface of Achaichia and Cowell and the corrugated fin surface of Davenport. Use of the j/f ratio as an approximate figure of merit led to an inaccurate assessment of the performance of the louvered fin heat exchanger surfaces evaluated in this study. Received on 8 May 1998     

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.     

Effect of fin thickness on air-side performance of herringbone wavy fin-and-tube heat exchangers

In the present study, the effects of fin thickness on the heat transfer and friction characteristics of fin-and-tube heat exchangers having herringbone wavy fin configuration are experimentally investigated. The experimental apparatus consists essentially of a well insulated open wind tunnel and herringbone wavy fin-and-tube heat exchangers made from aluminium plate finned, copper tube. Air and water are used to be working fluids in air-side and tube-side, respectively. A total of 10 samples of the fin-and-tube heat exchangers are tested. The experimental procedures are conducted by keeping the inlet water temperature at a pre-selected value, adjusting the water volumetric flow rate at a specific value and varying the air velocity. The results are presented as plots of the Colburn factor and friction factor against the Reynolds number based on the fin collar outside diameter (Re_Dc). From the results, it is found that for number of tube rows (N) = 2, the Colburn factor increases with increasing fin thickness. For N 4, the Colburn factor decreases with increasing fin thickness when Re_Dc < 1800, and increases with increasing fin thickness when Re_Dc > 2500. The friction factor increases with increasing fin thickness when fin pitch (F_p) 1.81 mm.     

Friction and Heat Transfer in a Laminar Separated Flow behind a Rectangular Step with Porous Injection or Suction

Results of a numerical study of a laminar separated flow behind a rectangular step on a porous surface with uniform injection or suction are described. Two cases are considered: an unconfined flow past a step and flow evolution in a confined channel (duct). It is shown that mass transfer on the surface causes strong changes in the flow structure and substantially affects the position of the reattachment point, as well as friction and heat transfer. More intense injection leads first to an increase in the separation-zone length and then to its rapid vanishing due to boundary-layer displacement. Vice versa, suction at high Reynolds numbers Re _s > 100 reduces the separation-zone length. The duct flow has a complicated distribution of friction and heat-transfer coefficients along the porous surface owing to the coupled effect of the transverse flow of the substance and changes in the main flow velocity due to mass transfer. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 1, pp. 18–28, January–February, 2006.     

Friction and forced convective heat transfer in a sintered porous channel with obstacle blocks

This work experimentally studies the flow characteristics and forced convective heat transfer in a sintered porous channel that filled with sintered copper beads of three average diameters ( 0.830, and 1.163 mm). The pressure drop and the local temperature measurements can be applied to figure out the distributions of the friction coefficient and the heat transfer coefficient. Three sintered porous channels differ in the arrangement of obstacle blocks. Model A has no obstacle. Models B and C have five obstacle blocks facing down and up, respectively, in a sintered porous channel. The range of experimental parameters, porosity, heat flux, and effect of forced convection are 0.370 ≤ ɛ ≤ 0.385, q=0.228, 0.872, 1.862 W/cm², and 200 ≤ Re _d ≤ 800. The permeability and inertia coefficient of each of the three sintered porous channels are analyzed. The results for Model A agree with those obtained by previous investigations in C _f distribution. The heat transfer of Model C exceeds that of Model A by approximately 20%. Finally, a series of empirical correlation equations were obtained for practical applications and engineering problems.     

Experimental investigation into vortex structure and pressure drop across microcavities in 3D integrated electronics

Hydrodynamics in microcavities with cylindrical micropin fin arrays simulating a single layer of a water-cooled electronic chip stack is investigated experimentally. Both inline and staggered pin arrangements are investigated using pressure drop and microparticle image velocimetry (μPIV) measurements. The pressure drop across the cavity shows a flow transition at pin diameter–based Reynolds numbers (Re _d ) ~200. Instantaneous μPIV, performed using a pH-controlled high seeding density of tracer microspheres, helps visualize vortex structure unreported till date in microscale geometries. The post-transition flow field shows vortex shedding and flow impingement onto the pins explaining the pressure drop increase. The flow fluctuations start at the chip outlet and shift upstream with increasing Re _d . No fluctuations are observed for a cavity with pin height-to-diameter ratio h/d = 1 up to Re _d ~330; however, its pressure drop was higher than for a cavity with h/d = 2 due to pronounced influence of cavity walls.     

Experimental investigation into the effect of heat transfer on compressible air stream in a duct

A converging nozzle-constant area parallel passage with an outer duct encasing the constant-area passage has been built for investigating the effect of heat transfer on subsonic flow of an air stream. It is concluded experimentally as can be predicted analytically that large quantities of heat are needed in order to accelerate very slow air stream (incompressible) to sonic conditions. It is observed experimentally as confirmed analytically, that the increase in Mach number with heat addition is associated with a decrease in the local static pressure along the axis of the duct. It could be concluded that any more heat added beyond thermal choking will be accompanied by a decrease in the mass flow rate of the compressible flowing air.Nomenclature A cross-sectional area of the duct - C _{P air} specific heat of air joules/kg. °K - C _d discharge coefficient - D duct diameter - d orifice diameter m - dA _d elemental lateral area of the duct - h overall heat transfer coefficient - h head across orifice, mm. - M Mach number - m _air mass flow rate of air - P local static pressure - P _b back pressure at duct outlet - P ₀₁ stagnation pressure at duct inlet - gas density - _u air density upstream of orifice - _q incremental heat flow - T local static temperature - T ₀₁ stagnation temperature at duct inlet - T _h hot water temperature - q heat added per kg of flowing air - V flow speed     

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.     

Effects of a turbulent wall jet on heat transfer over a non-confined backward-facing step

Experimental and numerical analysis of a turbulent wall jet on the heat transfer downstream of a non-confined backward-facing step are presented. Several configurations are studied to analyse the influence of the upstream flow and the height of the step. An infrared camera and a hot wire were used to visualize a temperature map and measure the instantaneous velocity, respectively. The main objective was to visualize and compare both the fluid flow and the heat transfer, by studying the skin friction coefficient C_f and the Nusselt number Nu_d, respectively. The latter is obtained by the calculation of the heat transfer coefficient, evaluated by inverse method. Both experimental data and numerical approach provide good agreement regarding the flow structure and thermal data for measuring the position and the value of characteristics scales in the recirculation zone. A correlation between the maximum heat transfer Nu_max and the maximum Reynolds number Re_max is presented. Similarities and differences are highlighted in the paper compared to confined configurations.     

Flow and heat transfer between plates with longitudinal fins

Laminar, fully developed flow and heat transfer between parallel plates with longitudinal fins are analyzed. A modified eigenfunction expansion and point-match method gives highly accurate results. The resistance productf Re and Nusselt numbers for bothH1 andH2 problems are determined as a function of fin length and spacing. It is possible to decrease both size and weight of the heat exchanger by the addition of fins.