Thermal performance assessment of turbulent channel flows over different shaped ribs

Experiments are conducted to assess turbulent forced convection heat transfer and friction loss behaviors for air flow through a constant heat flux channel fitted with different shaped ribs. The rib cross-sections used in the present study are triangular (isosceles), wedge (right-triangular) and rectangular shapes. Two rib arrangements, namely, in-line and staggered arrays, are introduced. Measurements are carried out for a rectangular channel of aspect ratio, AR = 15 and height, H = 20 mm with single rib height, e = 6 mm and rib pitch, P = 40 mm. The flow rate is in terms of Reynolds numbers based on the inlet hydraulic diameter of the channel in a range of 4000 to 16,000. The experimental results show a significant effect of the presence of the ribs on the heat transfer rate and friction loss over the smooth wall channel. The in-line rib arrangement provides higher heat transfer and friction loss than the staggered one for a similar mass flow rate. In comparison, the wedge rib pointing downstream yields the highest increase in both the Nusselt number and the friction factor but the triangular rib with staggered array shows better thermal performance over the others.     

Convection heat transfer from tube banks in crossflow: Analytical approach

The main objective of this analytical study is to investigate heat transfer from tube banks in crossflow under isothermal boundary condition. For this purpose, a control volume is selected from the fourth row of a tube as a typical cell to study the heat transfer from an in-line or staggered arrangement. An integral method of boundary layer analysis is employed to derive closed form expressions for the calculation of average heat transfer from the tubes of a bank, that can be used for a wide range of parameters including longitudinal pitch, transverse pitch, Reynolds and Prandtl numbers. The models for in-line and staggered arrangements are applicable for use over a wide range of parameters when determining heat transfer from tube banks.     

Thermal performance analysis of a tube finned surface

The present work submits an experimental work on the heat transfer and friction loss characteristic, employing a tube finned heating surface kept at a constant temperature in a rectangular channel. The tube fins attached on the surface (o.d.=29 mm) were arranged as either in‐line or staggered. The parameters for the study were Reynolds number (3700–30 000), depending on hydraulic diameter, the distance between the tube fins in the flow direction (S_y/D=1.72–3.45) and the fin arrangement. The change in the Nusselt number with these parameters was determined. For both tube fin arrangements, it was observed that increasing Reynolds number increased Nusselt number, and maximum heat transfer occurred at S_y/D=2.59. Thermal performances for both arrangements were also determined and compared with respect to heat transfer from the same surface without fins. With staggered array, a heat transfer enhancement up to 25 per cent for S_y/D=3.45 in staggered array was achieved in constant pumping power. Copyright © 2002 John Wiley & Sons, Ltd.     

Three-dimensional performance analysis of plain fin tube heat exchangers in transitional regime

Three-dimensional CFD simulations are carried out to investigate heat transfer and fluid flow characteristics of a four-row plain fin-and-tube heat exchanger using the Commercial Computational Fluid Dynamics Code ANSYS CFX 12.0. Heat transfer and pressure drop characteristics of the heat exchanger are investigated for Reynolds numbers ranging from 400 to 2000. Fluid flow and heat transfer are simulated and results compared using both laminar and turbulent flow models (k-ω) with steady and incompressible fluid flow. Model validation is carried out by comparing the simulated case friction factor (f) and Colburn factor (j) with the experimental data of Wang et al. [1]. Reasonable agreement is found between the simulations and experimental data. In this study the effect of geometrical parameters such as fin pitch, longitudinal pitch and transverse pitch of tube spacing are studied. Results are presented in the form of friction factor (f) and Colburn factor (j). For both laminar and transitional flow conditions heat transfer and friction factor decrease with the increase of longitudinal and transverse pitches of tube spacing whereas they increase with fin pitches for both in-line and staggered configurations. Efficiency index increases with the increase of longitudinal and transverse pitches of tube spacing but decreases with increase of fin pitches. For a particular Reynolds number, the efficiency index is higher in in-line arrangement than the staggered case.     

Experimental Study of Heat Transfer and Pressure Drop for H-type Finned Oval Tube with Longitudinal Vortex Generators and Dimples under Flue Gas

To enhance heat transfer and reduce fouling of the finned-tube surface in economizers of coal-fired power plants, heat transfer and pressure drop characteristics for H-type finned oval tube with longitudinal vortex generators (LVG) and dimples, both in-line and staggered arrangements, are studied experimentally under flue dust condition. In addition, the ash samples and heat exchanger surfaces after the test are analyzed to help understanding the ash fouling and tube wear mechanisms. Compared to the original H-type finned oval tube, the Nusselt number of H-type finned oval tube bank with longitudinal vortex generators and dimples is improved by 34.5–41.7% (in-line arrangement) and 28.1–31.7% (staggered arrangement) within the studied Reynolds numbers, while the Euler number is increased by 21.9–28.3% (in-line arrangement) and 19% (staggered arrangement) in the clean finned-tube surface state. In the stable fouling state, the Nusselt number is improved by 37.7–42.3% (in-line arrangement) and 27.8–45.1% (staggered arrangement), while the Euler number is increased by 22.9–25.2% (in-line arrangement) and 33.3–42% (staggered arrangement). The results show that the novel fin structures can both inhibit fouling and enhance heat transfer effectively.     

Thermal characterization of turbulent flow in a channel with isosceles triangular ribs

The article presents an experimental investigation on turbulent heat transfer and friction loss behaviors of airflow through a constant heat-fluxed channel fitted with different heights of triangular ribs. The rib cross-section geometry used in the present study was isosceles triangle. Two rib arrangements, namely, in-line and staggered arrays, were introduced. Measurements were carried out for a rectangular channel of aspect ratio, AR = 10 and height, H = 30 mm with three uniform rib heights, e = 4, 6 and 8 mm (e/H = 0.13, 0.2 and 0.26) and one non-uniform rib height, e = 4,6 mm (e/H = 0.13,0.2) alternately for a single rib pitch, P = 40 mm. The flow rate in terms of Reynolds numbers based on the inlet hydraulic diameter of the channel was in a range of 5000 to 22,000. The experimental results show a significant effect of the presence of the ribs on the heat transfer rate and friction loss over the smooth wall channel. The uniform rib height performs better than the corresponding non-uniform one. The in-line rib arrangement provides higher heat transfer and friction loss than the staggered one for a similar mass flow rate. In comparison, the largest e/H rib with inline array yields the highest increase in both the Nusselt number and the friction factor values but the lowest e/H rib with staggered array provides the best thermal performance.     

Measurement of endwall heat transfer and pressure drop in a pin-fin wedge duct

This paper discusses the measurements of endwall heat transfer and pressure drop in a wedge-shaped duct inserted with an array of circular pin fins. The endwall surface is coated with a thin layer of thermochromic liquid crystals and a transient test is run to obtain detailed heat transfer distributions. Parametric studies include Reynolds number (10,000?Re?50,000), outlet flow orientation (straight and lateral) and pin configuration (staggered and in-line). The wedge duct has a convergent angle of 12.7°. The pin spacing-to-diameter ratios along the longitude and transverse directions are fixed at s_x/d=s_y/d=2.5. Pin-less wedge duct results are also obtained for comparison. Results indicated that the straight wedge duct with a staggered pin array is most recommended because of its significant endwall heat transfer and moderate pressure-drop penalty; while the turned wedge duct with a staggered pin array is least recommended since it yields the highest pressure drops and raises severe hot spots. A similarity of the pin Reynolds-number dependence of row-averaged Nusselt number is developed in the present wedge duct of accelerating flow.     

Experimental Investigation of the Effect of Tube-to-Tube Porous Medium Interconnectors on the Thermohydraulics of Confined Tube Banks

This experimental study investigates the effect of tube-to-tube copper porous interconnectors on the thermohydraulic performance of an in-line and staggered confined tube bank. The porous medium, having a transverse thickness equal to that of the diameter of the tube (9 mm), connects longitudinally six successive tubes kept as in-line and staggered arrangements with a square pitch of 2.0. The tubes are subjected to a constant and uniform heat flux and are cooled by forced convection under laminar-transition flow range (200 < Reynolds number < 1500) using air with a Prandtl number of 0.71 as cooling fluid. Experimental data presented here establish that by introducing tube-to-tube porous medium interconnectors for the maximum Reynolds number tested here, a reduction in the pressure drop by 18% is observed in the in-line configuration while the heat transfer is enhanced by 100% in the staggered configuration, when compared to their respective configurations without the porous medium. Defining an overall energy gain as the ratio of the heat transfer enhancement due to the presence of the porous inserts to the pressure drop incurred, it is seen that fixing the porous inserts in the in-line configuration is advantageous.     

Investigation of heat transfer augmentation in a horizontal tube using different rectangular cut ring inserts

Experimental and computational investigations have studied the heat transfer, friction factor, and enhancement of heat transfer in a horizontal tube equipped with rectangular cut ring inserts and different diameter ratios (D/d) and pitch-to-tube diameter ratios (p/d_t). In the present study, air having a Reynolds no. range of 6700–20,100 was used as a working fluid. Three diameter ratios (D/d) were considered experimentally and numerically as 1.2, 1.25, and 1.3, and the pitch-to-tube diameter ratio (p/d_t) was (1, 0.625, and 0.5). Air was forced as working fluid through the tube and a uniform heat flux of 2000, 3500, and 5000 W/m² was applied through the tube's exterior surface. On the basis of the turbulence model k–ɛ with various parameters, three-dimensional numerical simulations using the ANSYS Fluent software 17.2 were investigated. Under the same working conditions, the results manifested a higher heat transfer rate and friction factor as compared to the plain tube. The results evinced that the Nusselt number for a horizontal tube equipped with rectangular cut ring inserts having various pitch ratios and diameter ratios is discovered to be higher than that for the plain tube. With the increased ring spacing, the overall improvement in heat transfer occurred. And, with a rise in Re, the total enhancement ratio decreased. Consequently, the greatest overall improvement attained was 38% at Reynolds number (Re = 12,860) with the pitch ratio (p/d_t = 1). The three diameter ratios (D/d) of 1.3, 1.25, and 1.2 gives in this study the average thermal performance factor in the value of 1.6, 1.5, and 1.4, respectively. Using the Nusselt number and friction factor, the results are correlated as a function of the Reynolds number, diameter ratio, and pitch ratio.     

Heat transfer augmentation in a wedge-ribbed channel using winglet vortex generators

Experimental investigations have been carried out to study the effect of combined wedge ribs and winglet type vortex generators (WVGs) on heat transfer and friction loss behaviors for turbulent airflow through a constant heat flux channel. To create a reverse flow in the channel, two types of wedge (right-triangle) ribs are introduced: wedge ribs pointing downstream and pointing upstream. The arrangements of both rib types placed inside the opposite channel walls are in-line and staggered arrays. To generate longitudinal vortex flows through the tested section, two pairs of the WVGs with the attack angle of 60° are mounted on the test channel entrance. The test channel has an aspect ratio, AR = 10 and height, H = 30 mm with a rib height, e/H = 0.2 and rib pitch, P/H = 1.33. The flow rate in terms of Reynolds numbers is based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The presence of the combined ribs and the WVGs shows the significant increase in heat transfer rate and friction loss over the smooth channel. The Nusselt number and friction factor values obtained from combined the ribs and the WVGs are found to be much higher than those from the ribs/WVGs alone. In conjunction with the WVGs, the in-line wedge pointing downstream provides the highest increase in both the heat transfer rate and the friction factor while the staggered wedge pointing upstream yields the best thermal performance.     

An asymptotic approach to generalizing the experimental data on convective heat transfer of tube bundles in crossflow

Based on the analysis of experimental data the universal methods of calculating convective heat transfer of smooth and finned tube bundles in the crossflow have been developed over the ranges of geometric characteristics covering all practical needs at the Reynolds number Re = 3 × 10³…1 × 10⁵.The distinctive feature of the methods proposed is that the generalized similarity equation of convective heat transfer takes into account the dependence of the Reynolds number exponent on tube pitch characteristics in a bundle. This has allowed the mechanism of heat transfer and hydraulic performance in tube bundles to be taken into utmost consideration and the asymptotic character to be given to the generalized dependence. In turn, this has permitted one to show the presence of maximum of heat transfer intensity and also to cover limiting combinations of pitches, at which differences in staggered and in-line arrangements of tubes are leveled, i.e., practically the restrictions on the ranges of tube pitch characteristics of bundles can be removed.     

On the combined influence of Reynolds number and cylinder spacing for flow around a row of heated square cylinders

The characteristics of forced convection heat transfer across a row of heated square cylinders kept in side-by-side arrangement are numerically investigated to examine the combined effects of Reynolds number and cylinder spacing for Ri = 0, 60 ≤ Re ≤ 160, Pr = .71, and s/d = 1.0–8.0, where the space between cylinder surfaces is s and the cylinder size is d. A numerical study was carried out using the thermal lattice Boltzmann method. The goal of this work is to explore the transitions in heat transfer phenomenon that occurs behind the cylinder and to report the corresponding regimes for heat transfer namely synchronous, quasiperiodic, and chaotic. The proposed regime of heat flow is a function of Reynolds number and spacing. The synchronous heat regime is obtained for s/d ≥ 5.0 and quasiperiodic, chaotic regimes are observed for 3.0 ≤ s/d < 5.0, s/d < 3.0, respectively at Re = 100. The instantaneous isotherms, the power spectra of the corresponding Nusselt number signals, and the significance of cylinder Nusselt number frequency are used to examine these heat flow regimes. The heat transfer regimes for a row of heated cylinders and flow regimes for a row of unheated cylinders both have comparable appearances except for the fact that the heat transfer regime is synchronous at s/d ≥ 5.0 and flow is synchronous at s/d ≥ 4.0. The chaotic or quasiperiodic heat transfer regimes occur due to merging and strong interactions between thermal blobs shed from the cylinders. Heat transfer is synchronous at a higher spacing and characterized by independent thermal blobs shedded from the cylinders. It is reported that as spacing reduces and Reynolds number increases, the mean value of the Nusselt number experienced by all cylinders increases. The important outcome of the present numerical work is that for understanding heat transfer from bluff body, the transitions that occur in heat transfer are useful.     

Experimental study of the convective heat transfer from in-line and staggered configurations of two wall-mounted cubes

The paper reports on the experimental investigation of the effects of the relative obstacle position on the convective heat transfer from a configuration of two wall-mounted cubes located in a fully developed turbulent channel flow. Both in-line and staggered arrangements were studied for various streamwise (S_x/H) and spanwise (S_z/H) distances. Distributions of the local heat transfer coefficient (h) were obtained from infrared thermography and local convective heat flux analyses. Laser Doppler anemometry measurements and flow visualisations were performed to document the flow and turbulence fields around the cubes. The results showed a large variation in the distribution of the local convective heat transfer for the various in-line and staggered configurations studied. While the in-line arrangements were featured by symmetric flow pattern and heat transfer distributions, the staggered arrangements showed distinct asymmetric pattern for certain combinations of S_x/H and S_z/H. Flow reattachment caused typically a monotonic decay of the convective heat transfer. On the other hand, flow separation caused distinct heat transfer extrema at the cube faces. In addition, the effect of vortex shedding on the convective heat transfer of the downstream cube was studied with a fast-responding heat flux sensor. Despite distinct variation in the distribution of the time-averaged heat transfer coefficient, the cube-averaged heat transfer coefficients appeared to be independent of the relative placement of the two cubes.     

Pressure drop and heat transfer of square pin-fin arrays in in-line and staggered arrangements

This work experimentally studied the pressure drop and heat transfer of a square pin-fin array in a rectangular channel by using the transient single-blow technique. The variable parameters are the relative longitudinal pitch (X_L = 1.5, 2, 2.8), the relative transverse pitch (X_T = 1.5, 2, 2.8) and the arrangement (in-line or staggered). Compared with the open articles, the present relative pitches are smaller and independently variable. The performance of the square pin-fins as the cooling devices is compared with that of the circular pin-fins. Besides, empirical formulas for the pressure loss and the heat transfer are suggested. Finally, the optimal inter-fin pitches are provided based on the largest Nusselt number under the same pumping power, while the optimal inter-fin pitches of square pin-fins are X_T = 2 and X_L = 1.5 for the arrays in in-line arrangements as well as X_T = 1.5 and X_L = 1.5 for the arrays in staggered arrangements.     

Numerical study of thermal-hydraulic and dust-deposition of tube bundles in an intermediate heat exchanger

Very-high-temperature gas-cooled reactors (VHTRs) are promising for efficient, zero-carbon hydrogen production. The intermediate heat exchanger (IHX) is a key piece of equipment for VHTR-coupled hydrogen production. Constrained by the limited space within the VHTR, the IHX must be efficient and compact. Besides, radioactive graphite dust deposition will reduce the performance and reliability of IHXs. Therefore, understanding the thermal-hydraulic and dust-deposition characteristics of IHX tube bundles is crucial for the design and safe operation of IHXs. This study uses the unsteady k-kl-ω model to simulate flow and heat transfer in IHX tube bundles. The discrete particle model combined with a deposition model is used to predict the movement and deposition behavior of graphite dust. The deposition model is achieved using user-defined functions. The model and code are first validated by empirical correlations and experimental results. The transient flow fields show that, in the in-line arrangement, unsteady and asymmetric periodic flow occurs with a period of about 0.047 s. The flow is steadier and more symmetric in the staggered arrangement due to the restriction of the main flow to the separation vortices. The maximum circumferential heat-transfer coefficient for inner tubes occurs at the impact point, where the central angle θ is about 60° and 0° for the in-line and staggered arrangement, respectively. The graphite-dust deposition rate decreases with increasing Reynolds number and particle size and the deposition mechanism is also analyzed in detail. With the current design conditions, the Nusselt number of the staggered arrangement increases by 27.90%–29.17% compared with the in-line arrangement, and the deposition rate decreases by 1.52%–3.15%. Furthermore, new correlations for Nusselt number and friction factor are developed for the thermal-hydraulic design of IHX tube bundles.     

Mixed Convection Heat Transfer from an In-Line Row of Square Cylinders in Cross-Flow at Low Reynolds Number

This article presents a two-dimensional numerical study on the unsteady laminar mixed convection heat transfer from a row of five in-line isothermal square cylinders placed in an unconfined medium and subjected to cross-flow of a Newtonian fluid at low Reynolds number (Re = 125). The hydrodynamic and thermal transport phenomena are captured for the separation ratios (spacing to cylinder size ratio, s/d) of 0.5, 1, 1.5, 2, 3, and 4. The mixed convection heat transfer is studied for Richardson numbers (Ri) ranging from 0 to 3 with a fixed Prandtl number (Pr = 0.71). Numerical calculations are performed by using a PISO algorithm-based finite volume solver in a collocated grid system. The instantaneous vorticity fields along with the isotherm patterns are systematically presented and discussed for different separation ratios and Richardson numbers. Depending on the engineering application, the temperature difference between the surface and the free stream could vary to make buoyancy of primary importance, entailing major modification of the flow field. Additionally, the instantaneous and mean drag and lift coefficients, Strouhal numbers, and mean Nusselt numbers are determined and discussed.     

Heat transfer and turbulent flow friction in a circular tube fitted with conical-nozzle turbulators

The paper deals with an experimental study of the influence of conical-nozzle turbulator inserts on heat transfer and friction characteristics in a circular tube. In the present work, the turbulators are placed in the test tube section with two different types: (1) diverging nozzle arrangement (D-nozzle turbulator) and (2) converging nozzle arrangement (C-nozzle turbulator). The turbulators are thoroughly inserted inside the tube with various pitch ratios, PR = 2.0, 4.0, and 7.0. The Reynolds number based on the bulk average properties of the air is in a range of 8000 to 18,000 and the experimental data obtained are compared with those obtained from the plain tube and from the literature. The experimental results reveal that increasing the Reynolds number at a given pitch ratio of the turbulators leads to the significant increase in Nusselt number indicating enhanced heat transfer coefficient due to rising convection as the flow increases. However, the friction factor at a given Reynolds number considerably increases with the reduction of pitch ratio and Reynolds number. The D-nozzle arrangement, creating stronger reverse/turbulence flow, provides higher the heat transfer rate and friction factor than the C-nozzle arrangement. The heat transfer rates obtained from using both nozzle-turbulators, in general, are found to be higher than that from the plain tube at a range of 236 to 344%, depending on Reynolds number and the turbulator arrangements. In addition, proposed correlations from the present experimental data for Nusselt number and friction factor are also presented.     

Experimental study of impinging heat transfer along rib-roughened walls by using transient liquid crystal technique

The objective of this work is to examine the detailed heat transfer coefficient distributions over a ribbed surface under impingement of in-line and staggered jet arrays by using a liquid crystal thermograph technique. In-line and staggered jet arrays with different exit flow orientations were considered. Three jet-to-target spacing Z of 3, 6 and 9 with in-line and staggered jet arrays were considered at jet Reynolds numbers of Re = 1500, 3000 and 4500 with three different exit flow orientations. In addition, the effects of rib configuration on the heat transfer distributions were discussed in detail. Results show that the local heat transfer rates over the ribbed surface are characterized by obvious periodic-type variation of Nusselt number distributions. The downstream peaks are diminished for increasing cross flow effect. Compared to the results without ribs, the heat transfer over the ribbed surface may be enhanced or retarded. Whereas, among the test angled-rib arrangements, the best heat transfer performance is obtained with a surface with 45° angled ribs.     

Thermal Performance Assessment of Turbulent Tube Flow Through Wire Coil Turbulators

This paper presents characteristics of turbulent convective heat transfer in a tube fitted with wire coil turbulators. Two different wire coils are introduced: (1) with typical/uniform coil pitch ratio (CR) and (2) with periodically varying three-coil pitch ratio. Various uniform coil pitch ratios (CR = 4, 6, and 8) and two periodically varying coil pitch ratios, the D-coil (decreasing three-coil pitch ratio arrangement) and DI-coil (decreasing/increasing three-coil pitch ratio arrangement), are experimentally investigated in a uniform heat flux tube. The experiments are performed for turbulent flows with Reynolds numbers ranging between 4500 and 20,000. All of the experimental results are compared with those obtained from using the plain tube, while the thermal performance factor is evaluated under an equal pumping power constraint. The experimental results show that the use of the tube fitted with all wire coils leads to an advantage on the basis of heat transfer enhancement over the plain tube with no insert. It is also observed that the uniform-pitch wire coil with higher coil pitch ratio (CR = 8) gives a higher thermal performance factor compared to ones with lower coil pitch ratios (CR = 4 and 6). In addition, for two periodically varying coil pitch ratios, the DI-coil performs with better heat transfer rate than the uniform-pitch ratio (CR = 6) and the D-coil for all Reynolds number ranges studied. The empirical correlations developed in terms of coil pitch ratios (CR), varying coil pitch ratios (D-coil and DI-coil), and Reynolds number are fitting the experimental data within plus or minus 3% and 5% for Nusselt number (Nu) and friction factor (f), respectively. The results of the thermal performance factor for various CR, D-coil, and DI-coil values are also determined.     

Influence of combined non-uniform wire coil and twisted tape inserts on thermal performance characteristics

In this paper, heat transfer, friction factor and thermal performance behaviors in a tube equipped with the combined devices between the twisted tape (TT) and constant/periodically varying wire coil pitch ratio are experimentally investigated. The periodically varying three coil pitch ratios were arranged into two different forms: (1) D-coil (decreasing coil pitch ratio arrangement) and (2) DI-coil (decreasing/increasing coil pitch ratio arrangement) while the twisted tapes were prepared with two different twist ratios. Each device alone is also tested and the results are subjected for comparison with those from the combined devices. The experiments were conducted in a turbulent flow regime with Reynolds numbers ranging from 4600 to 20,000 using air as the test fluid. Compared to each enhancement device, the heat transfer rate is further augmented by the compound devices. Over the range investigated, the highest thermal performance factor of around 1.25 is found by using DI-coil in common with the TT at lower Reynolds number. In addition, the empirical correlations of the heat transfer (Nu) and pressure drop (f) are also presented.