Experimental investigation of heat transfer and pressure drop in serrated-fin tube bundles with staggered tube layouts

An experimental investigation of the heat transfer and pressure drop performance of ten finned tube bundles using serrated fins is presented. All tube bundles had staggered layouts, and the influence on varying tube bundle layout, tube and fin parameters are presented. The heat transfer coefficient experienced a maximum when the flow areas in the transversal and diagonal planes were equal. An increase in the fin pitch increased the heat transfer coefficient; the same was observed with an increase in fin height. The pressure drop coefficient showed no influence of the tube bundle layout for small pitch ratios, but dropped significantly for higher ratios. Increasing fin pitch reduced the pressure drop, whereas varying fin height had insignificant effect. None of the literature correlations were able to reproduce the experiments for the entire range of tested conditions. A set of correlations were developed, reproducing the experimental data to within ±5% at a confidence interval of 95%.     

Heat transfer augmentation along the tube wall of a louvered fin heat exchanger using practical delta winglets

Delta winglets are known to induce the formation of streamwise vortices and increase heat transfer between a working fluid and the surface on which the winglets are placed. This study investigates the use of delta winglets to augment heat transfer on the tube surface of louvered fin heat exchangers. It is shown that delta winglets placed on louvered fins produce augmentations in heat transfer along the tube wall as high as 47% with a corresponding increase of 19% in pressure losses. Manufacturing constraints are considered in this study whereby piercings in the louvered fins resulting from stamping the winglets into the louvered fins are simulated. Comparisons of measured heat transfer coefficients with and without piercings indicate that piercings reduce average heat transfer augmentations, but significant increases still occur with respect to no winglets present.     

Comparisons of thermal performance and pressure drop of counterflow and parallel-flow heat-pipe heat exchangers with aligned/staggered tube rows

This paper presents a numerical calculation and comparison of thermal performance and pressure drop for heat-pipe heat exchangers with aligned/staggered tube rows. In this study, the variations of the number of rows and columns from 1 to 6 for a total 36 arrangements are presented and discussed. From the present results, an optimum arrangement of heat-pipe heat exchangers could be obtained.     

The effect of the number of tube rows on heat,mass and momentum transfer in flat-plate finned tube heat exchangers

The effect of the number of tube rows on heat, mass and momentum transfer is experimentally investigated for flat-plate, finned-tube heat exchangers which consist of aluminum fins and copper tubes. Four flat-plate finned-tube heat exchangers are identical except for changes in the number of tube rows (1 to 4). Heat-transfer coefficients for wet and dry surface conditions are obtained for both heating and cooling of moist air flowing over finned tubes. The air velocity was varied from 0.9 to 4 m/s. Heat transfer, Colburn and friction factors are determinated for different tube rows numbers while the Reynolds number were being warried. It is found that the values of Colburn and friction factors for wet surfaces are higher than for dry surfaces and for both conditions the Colburn and friction factors decrease with an increase in the tube row numbers.     

Heat transfer and pressure drop in a spirally grooved tube with twisted tape insert

In this paper, experimentally determined pressure drop and heat transfer characteristics of flow of water in a 75-start spirally grooved tube with twisted tape insert are presented. Laminar to fully turbulent ranges of Reynolds numbers have been considered. The grooves are clockwise with respect to the direction of flow. Compared to smooth tube, the heat transfer enhancement due to spiral grooves is further augmented by inserting twisted tapes having twist ratios Y ? 10.15, 7.95 and 3.4. It is found that the direction of twist (clockwise and anticlockwise) influences the thermo-hydraulic characteristics. Constant pumping power comparisons with smooth tube characteristics show that in spirally grooved tube with and without twisted tape, heat transfer increases considerably in laminar and moderately in turbulent range of Reynolds numbers. However, for the bare spiral tube and for spiral tube with anticlockwise twisted tape (Y = 10.15), reduction in heat transfer is noticed over a transition range of Reynolds numbers.     

Heat transfer and pressure drop characteristics in a circular tube with mesh cylinder inserts

Heat transfer performance and pressure drop tests were conducted on a circular tube with mesh cylinder inserts. These inserts with different open area rate, 30, 60, and 120 pores per inch (PPI) were tested at Reynolds numbers between 4000 and 18,000. Tap water was used as the working fluid. A high heat transfer coefficient was obtained by using mesh cylinder inserts. The friction factor increases with increases in PPI. Optimal results were obtained for PPI = 60. Heat transfer rates and friction factors were enhanced by 2.3–2.6 and 3.4–3.7 times, respectively, to those in the plain tube. Performance evaluation criterion (PEC) values were approximately 1.48–1.7. Numerical method is also employed to analyze the enhancement. The results show that the reason for the heat transfer enhancement is attributed to two factors. One is attributed to the velocity increase at the inlet. Another owes to the high turbulent intensity generated by inserting the mesh issue. Two factors work together to achieve a high heat transfer rate. Compared with other inserts, mesh cylinder inserts can transfer more heat for the same pumping power.     

Heat transfer and pressure drop performance of rod bundles arranged in square arrays

The report covers results of measurements of pressure drop and heat transfer performed on two rod bundles with 9 and 16 rods, respectively, in square arrays. The measurements were carried out with helium for Reynolds numbers between 10⁴ and 3× 10⁵. The rod bundles having a P/D ratio of 1.283 were operated at heat fluxes of up to 160W/cm and wall temperatures of up to 700°C. Pressure losses at the spacers and the circumferential temperature'distribution were also measured.The new measured results and the available data from the literature were used to elaborate relations for pressure drop and heat transfer in rod bundles with rods arranged in square arrays.     

Experimental study of heat transfer and flow of delta winglets inline arrays in a tube heat exchanger for enhanced heat transfer

This experiment was carried out using delta winglet arrays of vortex generators (VG) with inline arrangement in a tube heat exchanger to study enhanced heat transfer and flow behaviour. The experiment was conducted for the turbulent flow (Re = 6000 to 27000). In this experiment, different parameters, pitch ratios (PR = 1.6, 2.4, and 4.8), lengths (L = 10, 15, and 20 mm), and attack angles (B = 0°, 10°, 20°, 30°, and 45°) were studied and then their effect on thermal performance was observed. Results indicate that the PR affected f and Nu significantly. For PR = 1.6, VGs showed the highest f and Nu for all of the cases. Vortex generators with L10 B45 PR4.8 achieved the best TPE with 1.23 at Re = 6000. Attack angle B indicated a significant impact on thermal performance and 45 degree showed the TPE of 1.23 at lower Re. Oil film flow and smoke flow visualization were employed to identify the flow vortices and understand flow mechanism. The oil film flow and smoke flow visualization clearly traced longitudinal vortex, and induced vortex, which induced impingement flow and recirculation zone that lead to significant heat transfer enhancement.     

Heat transfer and pressure drop characteristics of peripheral-finned tube heat exchangers

The peripheral-finned tube is a new geometry aimed at avoiding moisture-condensate blockage hindering of the air-side heat transfer, by allowing for robust air flow pathways. It consists of a porous structure formed by periodic, radial-hexagonal fin arrangements of different radial extents mounted with a 30° offset from its neighboring level. Here, the air-side pressure drop and the heat transfer characteristics of five different heat exchanger prototypes with different geometric characteristics, such as the radial fin length, fin distribution, and heat exchanger length, were evaluated experimentally in an open-loop wind-tunnel calorimeter. The results demonstrate the effective performance, i.e., the pressure drop and heat transfer characteristics, of this new heat exchanger. A one-dimensional theoretical model based on the porous media treatment was also developed to predict the thermal-hydraulic behavior of the heat exchanger. The model incorporates the actual fin geometry into the calculation of the air-side porosity. The air-side permeability is calculated according to the Kozeny–Carman model and the particle-diameter based analysis. The model predicts the experimental data within a few percent RMS, depending on the correlations used for the friction coefficient and interstitial Nusselt number.     

Heat transfer and pressure drop in furrowed channels with transverse and skewed sinusoidal wavy walls

This comparative study examines the detailed Nusselt number (Nu) distributions, pressure drop coefficients (f) and thermal performance factors (η) for two furrowed rectangular channels with transverse and skewed sinusoidal wavy walls. Detailed heat transfer measurements over these transverse and skewed sinusoidal wavy walls at the Reynolds numbers (Re) = 1000, 1500, 2000, 5000, 10,000, 15,000, 20,000, 25,000 and 30,000 are performed using the steady-state infrared thermo-graphic method. Impacts of Re on Nu and f for two tested furrowed channels with transverse and skewed waviness are individually examined. In addition to the macroscopic mixing between the near-wall recirculations and core flows due to the shear layer instabilities in each wavy channel, the secondary flows tripped by the skewed wall-waves further elevate heat transfer performances and distinguish their Nu distributions from those over the transverse wavy wall. The area-averaged Nusselt numbers ($\overline{\mathit{Nu}}$) for two tested furrowed channels with transverse and skewed waviness with 5000 < Re < 30000 fall, respectively, in the ranges of 3.45–3.71 and 3.98–4.2 times of the Dittus–Boelter levels. A set of $\overline{\mathit{Nu}}$ and f correlations for each tested furrowed channel is individually derived using Re as the controlling parameter. By way of comparing the thermal performance factors (η) with a selection of rib-roughened channels, the η factors for the present skewed wavy channel are compatible with those in the channel roughened by the compound V-ribs and deepened scales due to the relative low pressure drop penalties with the equivalent heat transfer augmentations to those offered by V-ribs.     

Heat transfer and pressure drop correlations for laminar flow in an in-line and staggered array of circular cylinders

Enhanced heat transfer surfaces based on cylindrically shaped pin fins with wire diameters in the range of 100?µm were analyzed. The design is based on a high pin length to diameter ratio in the range of 20–100. Correlations for thermal and fluid dynamic characteristics of these fine wire structures are not available in literature. An in-line and staggered arrangement of pins were simulated for a variety of operational and geometrical conditions with a two-dimensional computational thermal and fluid dynamics model. Correlations for Nusselt number and friction factor with respect to Reynolds number and geometry were derived thereby. Reynolds numbers based on the wire diameter are in the range of 3–60. The correlations for the Nusselt number and friction factor can predict 93% and 97% of the simulated data within ±10%.     

Heat transfer and pressure drop correlations for the wavy fin and flat tube heat exchangers

A total of 11 cross-flow heat exchangers having wavy fin and flat tube were studied experimentally. A series of tests were conducted for air side Reynolds number in the range of 800–6500 with different fin pitches, fin lengths and fin heights, at a constant tube-side water flow rate of 2.5 m³/h. The air side thermal performance data were analyzed using the effectiveness-NTU method. The characteristics of heat transfer and pressure drop for different geometry parameters were reported in terms of Colburn j-factor and Fanning friction factor f, as a function of Re. The effects of fin pitch, fin height and fin length on the performance of heat transfer and pressure drop were examined. The general correlations for j and f factors were derived by multiple linear regression analysis and F test of significance. The correlations for j and f factors can predict 95% of the experimental data within ±10%.     

Experimental investigation of convective heat transfer and pressure loss in a round tube fitted with circular-ring turbulators

This paper presents the effect of the circular-ring turbulator (CRT) on the heat transfer and fluid friction characteristics in a heat exchanger tube. The experiments were conducted by insertion of CRTs with various geometries, including three different diameter ratios (DR = d/D = 0.5, 0.6 and 0.7) and three different pitch ratios (PR = p/D = 6, 8 and 12). During the test air at 27 °C was passed through the test tube which was controlled under uniform wall heat flux condition. The Reynolds number was varied from 4000 to 20,000. According to the experimental results, heat transfer rates in the tube fitted with CRTs are augmented around 57% to 195% compared to that in the plain tube, depending upon operating conditions. In addition, the results also reveal the CRT with the smallest pitch and diameter ratios offers the highest heat transfer rate in accompany with the largest pressure loss.     

Heat transfer and pressure loss characteristics of very compact heat sinks

High-performance compact heat sinks have been developed for the effective cooling of high-density LSI packaging. Heat transfer and pressure loss characteristics of the heat sinks in both air-cross-flow and air-jet cooling have been experimentally studied. The present heat sinks were of plate-fin and pin-fin arrays with a fin pitch of 0.7 mm. The plate-fin heat sinks had higher cooling performance than the pin-fin heat sinks in the range of large airflow rates both in air-cross-flow and air-jet cooling. The thermal conductance in cross-flow cooling was 20 or 40% larger than that in jet cooling. The correlation of Colburn j-factor/Fanning friction factor versus the Reynolds number for the present heat sinks was found to be very close to that of a conventional large-size heat exchanger. © Scripta Technica, Heat Trans Asian Res, 28(8): 687-705, 1999     

Heat transfer enhancement and pressure drop of the horizontal concentric tube with twisted wires brush inserts

In the present study, the heat transfer characteristics and the pressure drop of the horizontal concentric tube with twisted wires brush inserts are investigated. The inner diameters of the inner and outer tubes are 15.78 and 25.40 mm, respectively. The twisted wire brushes are fabricated by winding a 0.2 mm diameter of the copper wires over a 2 mm diameter of two twisted iron core-rod with three different twisted wires densities of 100, 200, 300 wires per centimeter. The plain tube with full-length twisted wires brush and regularly spaced twisted wires brush with 30 cm spacer length inserts are tested. Cold and hot water are used as working fluids in shell side and tube-side, respectively. The test runs are performed at the hot water Reynolds number ranging between 6000 and 20000. The inlet cold and hot water temperatures are 15, 20 °C, and between 40 and 50 °C, respectively. Effect of twisted wires density, inlet fluid temperature, and relevant parameters on heat transfer characteristics and pressure drop are considered. Twisted wire brushes insert have a large effect on the enhancement of heat transfer, however, the pressure drops also increase.     

Heat and mass transfer from a single horizontal tube of an evaporative tubular heat exchanger

In this investigation, water side heat transfer coefficients without air flow from a single horizontal tubes are determined. Mass transfer coefficients are determined with water and air flow from the same tube. The total energy dissipated by inside hot fluid when only water is falling is compared with that when both the air and water flow past the tube. The water side heat transfer coefficient and mass transfer coefficient are given by empirical relations h_w = 6.0(Re_p)^0.18(Re_w)^0.87 and K = 3.5(Re_p)^0.18(Re_a)^0.28 (Re_w)^0.54, respectively. The ratio of energies dissipated with water and air flow and with only water flow increases with Re_w and Re_a and its maximum value is 1.72 in the range of variables used.     

Heat transfer and pressure drop performance of twisted oval tube heat exchanger

Twisted oval tube heat exchanger is a type of heat exchanger that aims at improving the heat transfer coefficient of the tube side and also decreasing the pressure drop of the shell side. In the present work, tube side and shell side heat transfer and pressure drop performances of a twisted oval tube heat exchanger has been experimentally studied. The tube side study shows that the tube side heat transfer coefficient and pressure drop in a twisted oval tube are both higher than in a smooth round tube. The shell side study shows that the lower the modified Froude number Fr_M, the higher the shell side heat transfer coefficient and pressure drop. In order to comparatively analyze its shell side performance of the heat exchanger, a rod baffle heat exchanger with similar size of the twisted oval tube heat exchanger is designed and its performance is calculated with Gentry's method. The comparative study shows that the heat transfer coefficient of the twisted oval tube heat exchanger is higher and the pressure drop is lower than the rod baffle heat exchanger. In order to evaluate the overall performance of the twisted oval tube heat exchanger, a performance evaluation criterion considering both the tube side and shell side performance of a heat exchanger is proposed and applied. The analyze of the overall performance of the twisted oval tube shows that the twisted oval tube heat exchangers works more effective at low tube side flow rate and high shell side flow rate.     

Parametric study and multiple correlations on air-side heat transfer and friction characteristics of fin-and-tube heat exchangers with large number of large-diameter tube rows

In the present study, for industrial applications of large inter-coolers employed in multi-stage compressor systems the air-side laminar heat transfer and fluid flow characteristics of plain fin-and-tube heat exchangers with large number of tube rows and large diameter of the tubes are investigated numerically through three-dimensional simulations based on the SIMPLE algorithm in Cartesian coordinates. The effects of parameters such as Reynolds number, the number of tube rows, tube diameter, tube pitches and fin pitch are examined, and the variations of heat transfer due to variations of fin materials are also observed. It is found that the heat transfer and fluid flow approach fully developed conditions when the number of tube rows is greater than six, and the tube diameter as well as the fin pitch have much more significant effects than the tube pitch, and the heat transfer of high-conductivity material is larger than that of low-conductivity material especially in the high Reynolds number regime. Due to the fact that the existing correlations are not valid for large tube diameters and number of tube rows, the heat transfer and flow friction of the presented heat exchangers are correlated in the multiple forms. The correlation is so obtained that it can be used for further studies such as performance prediction or geometrical optimization.     

Heat transfer and pressure drop characteristics of laminar air flows moving in a parallel-plate channel with transverse hemi-cylindrical cavities

Laminar flows in parallel-plate channels are usually caused by a combination of small channel dimensions and low fluid velocities. As a consequence, the heat transfer coefficients in these channels are extremely low. The present study avoids inward protruding fins attached to the channel walls and instead focuses on the opposite arrangement. That is, molding the walls of parallel-plate channels with arrays of transverse cavities pointing outward. Two configurations were studied, one with symmetrically opposing cavities onto the bottom and upper walls and another with non-symmetric or staggered cavities onto the two parallel walls. A 120-cm-long channel contains two series of 3, 6 and 12 transverse cavities having ratios of cavity depth to cavity print diameter δ/D of 0.125, 0.25, 0.375, and 0.5. Computations are performed for Reynolds numbers based on the hydraulic diameter ranging from 1000 to 2500 for air (Pr = 0.7). The finite-volume method is used to perform the computational analysis with embedded second-order-accurate QUICK and SIMPLE schemes. It is found that the cavity/channel assemblies can achieve heat transfer enhancements of about 30% relative to the smooth channel, with pressure loss increases of 19%. In all cases examined, the outcome of the numerical simulation reveals that the heat transfer enhancement overcomes the pressure drop accretion.