Heat transfer,pressure drop,and fluid flow patterns in yawed tube banks

Complementary heat transfer, pressure distribution, and flow visualization experiments were performed to investigate the effect of yaw on both staggered and in-line tube tanks. The heat transfer measurements were carried out on a row-by-row basis, and pressures were measured internal to the tube banks as well as upstream and downstream. Air was the heat transfer fluid. The visualization experiments revealed that yaw markedly affected the manner in which the flow impinged on the tubes of the in-line array, with a lesser effect of yaw on the flow field in the staggered array. At a given freestream Reynolds number, the Nusselt number generally decreased as the angle of yaw increased. The yaw effect was well correlated for the staggered array, but not so well for the in-line array because of the aforementioned flow field modifications. The in-line-array Nusselt numbers generally exceeded those for the staggered array, a trend which was accentuated at larger yaw. The pressure drop decreased with increasing yaw. In the present operating range, the in-line-array pressure drops were smaller than the corresponding staggered-array values.     

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.     

Heat transfer and pressure loss penalty for the number of tube rows of staggered finned-tube bundles with a single transverse row of winglets

The objective of this research is to investigate the heat transfer and pressure loss penalty for various numbers of transverse rows in staggered finned-tube bundles with a single transverse row of the winglet pairs beside the front row of the tube bundles. Experiments were performed for two, three, four and five rows of staggered tube bundles. The pairs of winglets were placed with a heretofore-unused orientation for the purpose of augmentation of heat transfer and reduction of pressure loss penalty. This orientation is called as “common flow up” configuration. For three rows of tubes with a single transverse row of winglets beside the front row of the tubes, the heat transfer was augmented by 30-10%, and yet the pressure loss was reduced by 55-34% with the increase of the Reynolds number (based on two times channel height) from 350 to 2100. The reduction of the pressure loss penalty for three rows of tube bundles is the largest in comparison with the other numbers of rows.     

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.     

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.     

螺纹槽管错排管束的传热特性及流动阻力特性研究

1引言近年来,随着市场经济的发展,换热设备迫切需要节约能源、节省材料和降低成本的优化设计,因此强化传热技术受到了国内外的广泛重视。螺纹槽管是一种有效的强化换热管型,由于螺纹槽管的粗糙表面,可以有效地防止烟气在管内积灰而造成换热能力的下降,可以提高壁温以避免低温腐     

Performance predictions of laminar heat transfer and pressure drop in an in-line flat tube bundle using an adaptive neuro-fuzzy inference system (ANFIS) model

This paper shows how to predict the heat transfer and pressure drop for in-line flat tube configuration in a crossflow, using an adaptive neuro-fuzzy inference system (ANFIS). A numerical study of a 2D steady state and incompressible laminar flow for in-line flat tube configuration in a crossflow is also considered in this study. A finite volume technique and body-fitted coordinate system is used to solve the Navier–Stokes and energy equations. The Reynolds number varies from 10 to 320. Heat transfer and pressure drop results are presented for a tube configuration at transverse pitch and longitudinal pitch. The variation in velocity profile, isotherm contours and streamlines were compared for various configurations. The predicted results for average Nusselt number and dimensionless pressure show a good agreement with available previous work. The accuracy between numerical values and ANFIS model results were obtained with a mean relative error for average Nusselt number, pressure drop less than 1.9% and 2.97% respectively. Therefore, the ANFIS model is capable of predicting the performance of thermal systems in engineering applications, including the model of the tube bundle for heat transfer analysis and pressure drop.     

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.     

Heat transfers from pin-fin arrays experiencing forced convection

Steady-state heat-transfers from pin-fin arrays have been investigated experimentally for staggered and in-line arrangements of the pin fins, which were orthogonal to the mean air-flow. For the applied conditions, the optimal spacings of the fins in the span-wise and stream-wise directions have been determined. The dependences of the Nusselt number upon the Reynolds number and pin-fin pitch (in both directions) have been deduced.     

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.     

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.     

An experimental heat transfer study for helically flowing outside petal-shaped finned tubes with different geometrical parameters

The performances of shell-side heat transfer and pressure drop were experimentally studied in a helically baffled single tube heat exchanger, where water was used as a working medium. The tested tubes included one smooth tube and five petal-shaped fin tubes (PF tubes) with different geometrical parameters for improving the heat transfer of the shell side. It was shown that, compared with the smooth tube, five PF tubes significantly increased the values of Nusselt numbers. The Nusselt numbers increased with the fin height and decreased with the fin pitch. In the range of the present experiments, it was found that the Nusselt numbers for the PF tubes were increased by up to 233%, while the pressure drop was increased by less than 111%, as compared with that for the smooth tube. It is a promising route to use a PF tube instead of smooth tube for improving the performance of a helically baffled heat exchanger.     

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.     

Convective Heat Transfer and Pressure Drop over Elliptical and Flattened Tube

A numerical study on the effect of the effect of elliptical and flattened tube bundle geometry on the convective heat transfer and pressure drop is presented in this article. The analysis has been carried out to evaluate the performance of these bundle geometries in the design of a compact and effective single phase shell and tube heat exchanger. The temperature, velocity, and pressure drop profiles are obtained from solving the mass, momentum, and energy conservation equations. The comparison is made for inline and staggered bundle with different pitch to diameter ratio and inlet velocity for elliptical and flattened tubes. The pitch to diameter ratio is varied from 1.25 to 2.5 for Reynolds number ranging from 200 to 2000 which is in the laminar flow region. The heat transfer coefficient over the staggered and inline tube bundle decrease with an increase in pitch. The same kind of variation is also observed for the pressure drop in the case of both elliptical and flattened tube bundle. The study shows that the transverse pitch with respect to cross flow affects more than the longitudinal pitch.     

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.     

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.     

Influence of Tube Arrangement on the Thermal Hydraulic Performance of a Membrane Helical-Coil Heat Exchanger

Thermalhydraulic performances of membrane helical-coil heat exchangers in in-line and staggered arrangements, are numerically investigated. The influences of Reynolds number, dimensionless pitch, and arrangement on heat transfer and flow are discussed. Moreover, the axial and tangential velocity distributions were presented for various pitches and arrangements. The results show that the membrane helical-coil arrangement has a profound effect on the thermalhydraulic performance. For the smaller radial pitch (s ₁/d < 1.8), the heat transfer coefficient in in-line arrangement is higher than that in staggered arrangement. Meanwhile, the in-line arrangement gives a significantly higher friction factor than the staggered arrangement. However, for the larger radial pitch (s ₁/d ≥ 1.8), the coil arrangement appears to have no effect on the heat transfer and friction factor. In addition, the average Nusselt number Nu and the friction factor f for in-line and staggered arrangements were calculated and correlated against the Reynolds number and structural parameters. According to the thermalhydraulic performance evaluation criterion, the staggered arrangement is recommended as the optimal coil arrangement.     

Local heat transfer characteristics of array impinging jets from elongated orifices

The aim of this research is to enhance the heat transfer on an impinged surface under an impinging jet array by minimizing a cross-flow effect. Conventional round orifices (aspect ratio, AR = 1) are substituted by the elongated orifices with aspect ratio AR = 4 and 8 with the same jet exit area. Two types of orifice arrangements; in-line and staggered arrays are compared. The experimental investigation was carried out at constant distance from orifice plate to impinged surface H = 2D_E (D_E is equivalent diameter of orifice). The heat transfer characteristic was visualized using thermochromic liquid crystal sheet (TLCs) and the Nusselt number distribution was evaluated by an image processing technique. The flow characteristic on the impinged surface was also visualized by oil film technique. The results show that the cross-flow in a case of the jets issued from the orifices with AR = 4 is considerably less significant than that in cases of the ones delivered from the orifices with AR = 1 and 8. At Reynolds number of 13,400, the Nusselt numbers for the jet arrays issued from the elongated orifices with AR = 4 with in-line and staggered arrangements are respectively 6.04% and 12.52% higher than those for the case of AR = 1.     

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.     

Etude des structures convectives et du transfert thermique autour d'un cylindre en deuxième nappe dans une configuration en quinconce,en écoulement d'air chargé de gouttelettes d'eau

We describe an experimental study of the influence of longitudinal and transversal pitch on the convective transfer around a cylinder on the second row of a three cylinder staggered bank, operating in an airstream charged with sprayed water at moderate Reynolds number. The analysis of the wall velocity gradient and its fluctuation shows the effects of geometrical parameters on various characteristic zones of the flow: laminar boundary layer, separation and vortices. The modifications of local Nusselt number evolution compared with the single cylinder are explained. For the disposition which gives the highest heat transfer, a correlation of local Nusselt number is proposed.