Roughness enhanced mechanism for turbulent convective heat transfer

The mechanism of turbulent convective heat transfer enhancement was experimentally investigated by measuring the heat transfer in two dimensional roughness tubes with different roughness heights at various Reynolds numbers. The results show that there is a maximum Nusselt number ratio (Nu/Nu₀) for a fixed roughness height with increasing Reynolds numbers. For water as working fluid, heat transfer can hardly be increased when the roughness height is lower than the thickness of the viscous sublayer, and both heat transfer and flow friction begin to increase when the roughness height is higher than the viscous sublayer. When the roughness height is more than five times of the viscous sublayer thickness, the flow friction begins to increase sharply but heat transfer is slowly enhanced. So the best heat transfer enhancement for a given pumping power is reached when the roughness height is about three times of the viscous sublayer thickness. The Prandtl number influences to the turbulent heat transfer enhancement by roughness were also analyzed.     

Heat transfer and friction factor correlations for a duct having dimple-shape artificial roughness for solar air heaters

The heat transfer coefficient between the absorber plate and air can be considerably increased by using artificial roughness on the underside of the absorber plate of a solar air heater duct. Under the present work, an experimental study has been carried out to investigate the effect of roughness and operating parameters on heat transfer and friction factor in a roughened duct provided with dimple-shape roughness geometry. The investigation has covered the range of Reynolds number (Re) from 2000 to 12,000, relative roughness height (e/D) from 0.018 to 0.037 and relative pitch (p/e) from 8 to 12. Based on the experimental data, values of Nusselt number (Nu) and friction factor (f_r) have been determined for different values of roughness and operating parameters. In order to determine the enhancement in heat transfer and increment in friction factor values of Nusselt number and friction factor have been compared with those of smooth duct under similar flow conditions. Correlations for Nusselt number and friction factor have been developed for solar air heater duct provided such artificial roughness geometry.     

Development of correlations for Nusselt number and friction factor for solar air heater with roughened duct having arc-shaped wire as artificial roughness

An experimental study has been carried out for enhancement of heat transfer coefficient of a solar air heater having roughened air duct provided with artificial roughness in the form of arc-shape parallel wire as roughness element. Increment in friction factor by provided with such artificial roughness elements has also been studied. The effect of system parameters such as relative roughness height (e/d) and arc angle (α/90) have been studied on Nusselt number (Nu) and friction factor (f) with Reynolds number (Re) varied from 2000 to 17000. Considerable enhancement in heat transfer coefficient has been achieved with such roughness element. Using experimental data correlations for Nusselt number and friction factor have also been developed for such solar air heaters, which gives a good agreement between predicted values and experimental values of Nusselt number and friction factor.     

Exergy transfer in a porous rectangular channel

Present paper is performed to investigate the heat and exergy transfer characteristics of forced convection flow through a horizontal rectangular channel where open-cell metal foams of different pore densities such as 10, 20 and 30 PPI (per pore inches) were situated. All of the bounding walls of the channel are subjected to various uniform heat fluxes. The pressure drop and heat transfer characteristics are presented by two important parametric values, Nusselt number (Nu_H) and friction factor (f), as functions of Reynolds number (Re_H) and the wall heat flux (q). The Reynolds number (Re_H) based on the channel height of the rectangular channel is varied from 600 to 33?000, while the Grashof number (Gr_Dh) ranged from approximately 10⁵–10⁷ depending on q. Based on the experimental data, new empirical correlations are constructed to link the Nu_H. The results of all cases are compared to that of the empty channel and the literature. It is found that the results are in good agreement with those cited in the references. The mean exergy transfer Nusselt number (Nu_e) based on the Re_H, Nu_H, Pr and q for a rectangular channel with constant heat flux is presented and discussed.     

Thermal characteristics of turbulent rib-grooved channel flows

Experimental work has been performed to examine the combined effects of rib-grooved turbulators on the turbulent forced convection heat transfer and friction characteristics in a rectangular duct under a uniform heat flux boundary condition. In the experiments, three types of rib-groove arrangements: rectangular-rib and triangular-groove (RR–TG), triangular-rib and rectangular-groove (TR–RG) and triangular-rib with triangular-groove (TR–TG), were examined. Measurements were carried out for the duct of one aspect ratio, AR = W/H = 20 and duct height, H = 9 mm with rib height, e = 3 mm at three pitch ratios, PR = P/e = 6.6, 10 and 13.3. Experiments were conducted for the Reynolds number range of 3000 to 10,000. Influences of rib-groove arrangements on the Nusselt number and friction factor have been discussed and compared with smooth duct results under similar test conditions. Isothermal friction factors were also taken and presented. The obtained results of the smooth duct are in good agreement with the previous studies found in the literature. Experimental results also show that the duct with RR–TG arrangement provides maximum heat transfer rate and friction factor than others. On the other hand, the thermal enhancement index obtained at constant pumping power reveals that the TR–TG provides the highest values for all pitch ratios studied. Finally, correlations for the heat transfer (Nu), friction factor (f) and the enhancement index (η) have been developed as a function of pitch ratio (PR) and Reynolds number (Re).     

Heat transfer enhancement of transverse ribs in circular tubes with consideration of entrance effect

The heat transfer enhancement of transverse ribs in circular tubes with a length-to-diameter ratio of 87 was experimentally investigated. The mean heat transfer and friction data were obtained for the air flow started from the entrance. An isothermal surface condition was considered. The rib pitch-to-tube diameter ratio (p/d) was in the range 0.304–5.72; the rib height-to-tube diameter ratio (e/d) was in the range 0.015–0.143; the considered Reynolds number (Re) was in the range 4608–12,936. The mean Nusselt number (Nu) and friction factor (f) were individually correlated as a function of p/d, e/d and Re. A critical e/d, equal to 0.057, was found. For e/d < 0.057, the f is proportional to e/d; for e/d ⩾ 0.057, the f is proportional to (e/d)^2.55. A performance map, indicating the corresponding heat transfer enhancement index (r₁) and efficiency index (r₂) for various p/d and e/d, was constructed. This performance map clearly indicates the ranges of p/d and e/d with a high r₂.     

Thermal performances of enhanced smooth and spiky twisted tapes for laminar and turbulent tubular flows

This experimental study investigates the heat transfer properties over developing and developed flow regimes, the pressure drop coefficients and the thermal performance factors (TPF) of tubular flows with the continuous and spiky twist tapes enhanced by perforated, jagged and notched winglets. The axial distributions of Nusselt number (Nu) and the mean Fanning friction factors (f) of the tubular flows at Reynolds numbers (Re) ranging from 500 to 40000 are comparatively examined for five different types of twisted tapes with three twist ratios (y) of 1.875, 2.186 and 2.815 for each type of twisted tapes. Through this comparative study, the favorable types of twisted tapes which generate the higher degrees of HTE impacts over the developing and developed flow regimes are respectively identified. These newly devised twist tapes enrich the varieties of passive heat transfer enhancement (HTE) devices, especially for retrofit applications. A set of selective Nu and f results illustrates the thermal characteristics of the enhanced tubular flows by these twisted tapes. The HTE and TPF properties for all the present types of twisted tapes are subsequently compared with those reported for other types of twisted tapes in the literature. Among these comparative groups, the present V-notched spiky twisted tape generally offers the highest HTE impacts with favorable TPF performances. Empirical correlations that evaluate the averaged Nu over the developing and developed flow regimes; as well as and tube-wise averaged f for the enhanced tubular flows fitted with all the present types of twisted tapes are generated.     

Development of correlations for Nusselt number and friction factor for solar air heater with roughened duct having multi v-shaped with gap rib as artificial roughness

This paper presents the results of an experimental investigation of heat transfer and friction in the flow of air in rectangular ducts having multi v-shaped rib with gap roughness on one broad wall. The investigation encompassed Reynolds number (Re) from 2000 to 20,000, relative gap distance (G_d/L_v) values of 0.24–0.80, relative gap width (g/e) values of 0.5–1.5, relative roughness height (e/D) values of 0.022–0.043, relative roughness pitch (P/e) values of 6–12, relative roughness width ratio (W/w) values of 1–10, angle of attack (α) range of 30°–75°. The optimum values of geometrical parameters of roughness have been obtained and discussed. For Nusselt number (Nu), the maximum enhancement of the order of 6.74 times of the corresponding value of the smooth duct has been obtained, however the friction factor (f) has also been seen to increase by 6.37 times of that of the smooth duct. The rib parameters corresponding to maximum increase in Nu and f were G_d/L_v = 0.69, g/e = 1.0, e/D = 0.043, P/e = 8, W/w = 6 and α = 60°. Based on the experimental data, correlations for Nu and f have been developed as function of roughness parameters of multi v-shaped with gap rib and flow Reynolds number.     

Heat transfer and pressure drop in corrugated channels

The convective heat transfer and pressure drop characteristics of flow in corrugated channels have been experimentally investigated. Experiments were performed on channels of uniform wall temperature and of fixed corrugation ratio over a range of Reynolds number, 3220 ≤ Re ≤ 9420. The effects of channel spacing and phase shift variations on heat transfer and pressure drop are discussed. Results of corrugated channels flow showed a significant heat transfer enhancement accompanied by increased pressure drop penalty. The average heat transfer coefficient and pressure drop enhanced by a factor of 2.6 up to 3.2 and 1.9 to 2.6 relative to those for parallel plate channel, respectively, depending upon the spacing and phase shift. The friction factor increased with increasing channel spacing and its phase shift. The effect of spacing variations on heat transfer and friction factor was more pronounced than that of phase shift variation, especially at high Reynolds number. Comparing results of the tested channels by considering the flow area goodness factor (j/f), it was better for corrugated channel with spacing ratio, ? ≤ 3.0 and of phase shift, Ø ≤ 90°. Comparisons of the present data with those available in literature are presented and discussed.     

Experimental investigation on heat transfer and fluid friction correlations for circular tubes with coiled-wire inserts

Heat transfer and pressure drop data for air flow and water flow in smooth tubes with coiled-wire inserts were measured. The wire diameter-to-tube inner diameter ratio (e/d) and coil pitch-to-tube inner diameter ratio (p/d) are in the ranges of 0.0725 to 0.134 and 1.304 to 2.319 respectively. It is found that the Nusselt number (Nu) increases with the e/d value, whereas it increases with a decrease of the p/d value. As air is the working fluid, the dependence of the heat transfer enhancement of the wire coil on the Reynolds number (Re) is minor; as water is the working fluid, the heat transfer enhancement considerably decreases with an increase of the Re value. Two heat transfer empirical equations, one for air and the other for water, are proposed. For both air and water, a common fluid friction empirical equation is established. To effectively and efficiently enhance the heat transfer, for air, the proposed e/d and p/d values of the wire coil are 0.101 and 2.319, respectively; for water, the proposed e/d and p/d values are 0.101 and 1.739, respectively.     

Numerical study of fluid flow and heat transfer in a flat-plate channel with longitudinal vortex generators by applying field synergy principle analysis

Three dimensional numerical simulations are performed on laminar heat transfer and fluid flow characteristics of a flat-plate channel with longitudinal vortex generators (LVGs). The effects of two different shaped LVGs, rectangular winglet pair (RWP) and delta winglet pair (DWP) with two different configurations, common-flow-down (CFD) and common-flow-up (CFU), are studied. The numerical results indicate that the application of LVGs effectively enhances heat transfer of the channel. According to the performance evaluation parameter, (Nu/Nu₀)/(f/f₀), the channel with DWP has better overall performance than RWP; the CFD and CFU configurations of DWP have almost the same overall performance; the CFD configuration has a better overall performance than the CFU configuration for RWP. The basic mechanism of heat transfer enhancement by LVGs can be well described by the field synergy principle.     

Correlations for solar air heater duct with V-shaped perforated baffles as roughness elements on absorber plate

An experimental investigation has been carried out to study the effect of heat transfer and friction characteristics of air passing through a rectangular duct which is roughened by V-down perforated baffles. The experiment encompassed Reynolds number (Re) from 3800 to 19,000, relative roughness height (e/H) values of 0.285–0.6, relative roughness pitch (P/e) range of 1–4 and open area ratio values from 12% to 44%. The effect of roughness parameters on Nusselt number (Nu) and friction factor (f) has been determined and increase in heat transfer and friction loss has been observed for ducts having a roughened test plate. Maximum Nusselt number is observed for the relative roughness pitch ranging from 1.5 to 3 for flow and geometrical parameters under consideration. The experimental data have been used to develop Nusselt number and friction factor correlations as a function of roughness and flow parameters.     

Heat transfer and friction behaviors in rectangular channels with varying number of ribbed walls

An experimental study of surface heat transfer and friction characteristics of a fully developed turbulent air flow in a square channel with transverse ribs on one, two, three, and four walls is reported. Tests were performed for Reynolds numbers ranging from 10,000 to 80,000. The pitch-to-rib height ratio, P/e, was kept at 8 and rib-height-to-channel hydraulic diameter ratio, e/D_h was kept at 0.0625. The channel length-to-hydraulic diameter ratio, L/D_h, was 20. The heat transfer coefficient and friction factor results were enhanced with the increase in the number of ribbed walls. The friction roughness function, R(e⁺), was almost constant over the entire range of tests performed and was within comparable limits of the previously published data. The heat transfer roughness function, G(e⁺), increased with roughness Reynolds number and compared well with previous work in this area. Both correlations could be used to predict the friction factor and heat transfer coefficient in a rectangular channel with varying number of ribbed walls. The results of this investigation could be used in various applications of turbulent internal channel flows involving different number of rib roughened walls.     

Turbulent convection in round tube equipped with propeller type swirl generators

This article is aimed at studying the heat transfer, friction loss and enhancement efficiency behaviors in a heat exchanger tube equipped with propeller type swirl generators at several pitch ratios (PR). The investigation is performed for the Reynolds number ranging from 4000 to 21,000 under a uniform heat flux condition. The experiments are also undertaken for several blade numbers of the propeller (N = 4, 6, and 8 blades) and for different blade angles (θ = 30°, 45°, and 60°). The influences of using the propeller rotating freely, on heat transfer enhancement, pressure loss, and enhancement efficiency, are reported. In the experiments, the swirl generator is used to create a decaying swirl in the tube flow. Average Nusselt numbers are determined and also compared with those obtained from other similar cases. The experimental results indicate that the tube with the propeller inserts provides considerable improvement of the heat transfer rate over the plain tube around 2.07 to 2.18 times for PR = 5, blade angles θ = 60° and N = 8. The use of the propeller leads to maximum enhancement efficiency up to 1.2. Thus, because of strong swirl or rotating flow, the propellers and their blade numbers become influential upon the heat transfer enhancement. The increase in friction factor from using the propeller is found to be 3–18 times over the plain tube. Correlations for Nusselt number (Nu) and friction factor (f) for the inserted tube are provided and the performance evaluation criterion to access the real benefits in using the swirl generators of the enhanced tube is also determined.     

Convective heat transfer in a circular tube with short-length twisted tape insert

This work presents an experimental study on the mean Nusselt number, friction factor and enhancement efficiency characteristics in a round tube with short-length twisted tape insert under uniform wall heat flux boundary conditions. In the experiments, measured data are taken at Reynolds numbers in a turbulent region with air as the test fluid. The full-length twisted tape is inserted into the tested tube at a single twist ratio of y/w = 4.0 while the short-length tapes mounted at the entry test section are used at several tape length ratios (LR = l_s/l_f) of 0.29, 0.43, 0.57 and 1.0 (full-length tape). The short-length tape is introduced as a swirling flow device for generating a strong swirl flow at the tube entry before decaying along the tube. On the other hand, the full-length tape (LR = 1.0) is expected to produce a strongly swirling flow over the whole tube. The variation of heat transfer and pressure loss in the form of Nusselt number (Nu) and friction factor (f) respectively is determined and depicted graphically. The experimental result indicates that the short-length tapes of LR = 0.29, 0.43 and 0.57 perform lower heat transfer and friction factor values than the full-length tape around 14%, 9.5% and 6.7%; and 21%, 15.3% and 10.5%, respectively. In addition, it is apparent that the enhancement efficiency of the tube with the short-length tape insert is found to be lower than that with the full-length one. The mean deviation between measured and correlated values of the Nusselt number is in the order of ± 7% in the range of Reynolds numbers from 4000 to 20,000.     

Hydrogen flow and heat transfer characteristic analysis in cooling channel wall with the spherical convexity structure

In the thermal protection design for rocket engines chamber, the internal wall always suffers from high combustion temperature and small cooling channel size, thus an efficient method by changing inner wall structure in cooling channel is investigated. This paper proposed a structure with spherical convexity on the inner wall for improving the entire cooling performance, weakening thermal stratification phenomenon, and protecting the overheated structure. Optimized spacing is obtained through comparison to achieve better cooling effect and smaller flow pressure drop. An enhanced disturbance, which stems from the newly introduced spherical convexity surface to control coolant flow, finally improves the convective heat transfer efficiency. The results show that the spherical convexity structure of a depth 0.2 mm and spacing 2.5 mm is proper for enhancing heat transfer and weakening thermal stratification phenomenon in the channel with cross-section 2 mm*2 mm. Compared to the smooth channel, the (Nu/Nu₀)/(f/f₀)^1/3 is increased by 55%, and the maximum temperature is decreased by 10% but the pressure drop is only increased by a maximum 14.5%.     

Effect of apex angle variation on thermal and hydraulic performance of roughened triangular duct

The experiments were performed to analyze the influence of apex angle on thermal and hydraulic characteristics of triangular duct for Reynolds number range from 2000 to 16,000. Four different triangular ducts of apex angle 30°, 60°, 90° and 110° were fabricated for the experimentation. The one side of the duct is roughened with dimple shaped roughness element. The both relative shortway length (s/e) and relative longway length (l/e) of dimple shaped roughened element was kept constant (i.e., 10) but relative roughness height (e/D) is varied from 0.016 to 0.038. Result shows that apex angle plays an important role in heat transfer. The experimental results are presented in a form of correlation for Nusselt number (Nu) and friction factor (f).     

Heat transfer and friction in solar air heater duct with W-shaped rib roughness on absorber plate

Artificial roughness in form of ribs is convenient method for enhancement of heat transfer coefficient in solar air heater. This paper presents experimental investigation of heat transfer and friction factor characteristics of rectangular duct roughened with W-shaped ribs on its underside on one broad wall arranged at an inclination with respect to flow direction. Range of parameters for this study has been decided on basis of practical considerations of system and operating conditions. Duct has width to height ratio (W/H) of 8.0, relative roughness pitch (p/e) of 10, relative roughness height (e/D_h) 0.018-0.03375 and angle of attack of flow (α) 30-75°. Air flow rate corresponds to Reynolds number between 2300-14,000. Heat transfer and friction factor results have been compared with those for smooth duct under similar flow and thermal boundary condition to determine thermo-hydraulic performance. Correlations have been developed for heat transfer coefficient and friction factor for roughened duct.     

Numerical study on heat transfer enhancement for laminar flow in a tube with mesh conical frustum inserts

Enhanced heat transfer tubes (EHTT) with segmented mesh-conical frustums are considered. Tube diameter and frustum apex angle are fixed as 20?mm and 60o, respectively. The height ratio of frustum and sliced part are set as a golden ratio (1.618). Laminar thermal-hydraulic performance and effects of some parameters, e.g., bottom frustum diameter and pitch, are numerically simulated. The equal equivalent diameter and total flow area criteria are adopted to simplify 3D mesh pores to 2D ones. Flow and temperature fields show large velocities and gradients close to the wall and smaller velocities in the bulk region. This enhances heat transfer with a limited pressure drop. EHTTs obtain 1.4 - 3.3 times higher heat transfer than bare tubes and the performance evaluation criterion (PEC) varies from 1.3 to 1.8. Nusselt number (Nu) and friction factor (f) correlations are proposed. New insights into heat transfer enhancement and tube configuration are provided.     

Heat transfer and pressure drop correlations for compact heat exchangers with multi-region louver fins

An experimental study on the air-side heat transfer and pressure drop characteristics for brazed aluminum heat exchangers with multi-region louver fins and flat tubes was conducted. A series of tests were conducted for heat exchangers with different numbers of louver regions at the air-side Reynolds numbers of 400–1600 based on the louver pitch. The air-side thermal performance data were analyzed by using the effectiveness-NTU method. The characteristics of the heat transfer and pressure drop for heat exchangers with different geometry parameters were presented in terms of the Colburn j factor and Fanning friction f factor as function of the Reynolds number.