Heat Transfer Characteristics Of Tube Research Articles

Numerical study of different Pr number medium turbulent cross flow in staggered tube bundle

Tube bundle heat exchangers have been recognized as the most efficient means of thermal exchange. The staggered row method can enhance the performance of a heat exchanger. However, it results in a large pressure drop loss. As working media with high thermal conductivities, liquid metals can potentially replace conventional working media. In this study, for working fluids with low Prandtl number cross flowing through a staggered tube bundle, the turbulent Prandtl number model of cross flow was verified. The unsteady characteristics of the two working fluids traversing through the tube bundle were analyzed. The effects of molecular thermal diffusion on the total heat transfer capacity of the two working fluids during heat transfer were compared, along with the changes in the circumferential heat transfer characteristics of the tube bundles under different Reynolds numbers. The ζ factor was then proposed to analyze the non-uniformity in the heat transfer characteristics of a single tube. An empirical correlation describing the heat transfer non-uniformity at the fully developed region and inlet region and involving parameters such as working properties and flow state was proposed based on computational fluid dynamics data. Finally, the performance evaluation criteria(PEC) factor was used to analyze the comprehensive performance of the two heat transfer fluids. The results revealed that the comprehensive heat transfer performance of mercury was 60–70 times that of conventional working fluids.

Friction and Forced Convection Heat Transfer Characteristics in Tubes by Use of Helical-Wire-Coil Inserts.

This work summarizes the results of experimental investigation of convective heat transfer and friction loss characteristics of air flow in a horizontal circular copper tube of diameters 13.5/22 mm., using helical wire-coils , of diameter 1.4 mm, inserted in. The helix angle (a) of the helical-wire-coil is varied from 65° to 86.7o. The outer surface of the test tube is heated by superheated steam (degree of superheat 2:3 oC) at atmospheric pressure. During the experimental work, Reynold's number is changed from 6000 to 120000 and Prandti number is about 0.7. The heat transfer enhancement of the inserted helical-wire-coil is ranging from 30 % to 145 %. However, A quantitatively increase in hydrodynamics resistance is noticed. Correlations of Nusselt number and friction factor has been made. It was found that the nusselt number and friction factor were dependent on both Reynold's number and helix angle.

Numerical Analysis of FLiBe Laminar Convective Heat Transfer Characteristics in Tubes Fitted With Coaxial Cross Twisted Tape Inserts

Molten salts are now widely used as heat transfer fluids in solar thermal plants and molten salt nuclear reactors. In order to explore its heat transfer enhancement method, new coaxial cross twisted tapes (CCTTs) are applied to plain tube with molten salt FLiBe as working fluid, and the characteristics of pressure drop and heat transfer are numerically investigated by the CFD software STAR-CCM+. Simulations are performed in a laminar flow regime where Reynolds number ranges from 100 to 1100, and the velocity, temperature profiles, and enhanced performance evaluation criteria (PEC) are analyzed to investigate the heat transfer performance of FLiBe in the tube fitted with CCTTs and typical twisted tape (TT). Water and lubricating oil are also researched to investigate the effects of fluid thermal-physical properties on the performance of CCTTs. The results show that CCTTs can produce stronger swirl flow and greatly enhance heat transfer. For overall heat transfer performance, the maximum PEC of FLiBe with CCTTs reaches 2.37. The comparison of three different fluids also indicates that CCTTs have better heat transfer performance for higher Prandtl number fluids such as molten salt. The correlations of CCTTs and TT are developed into a unified form for the prediction of friction factors and Nusselt numbers for various Prandtl number fluids.

Numerical investigations on the heat transfer characteristics of tube in tube helical coil heat exchanger

Heat transfer characteristics of a tube in tube helical heat exchanger in the laminar flow regime is analysed numerically. Numerical scheme is validated with the experimental results reported in literature. The effect of helix angle on the overall heat transfer coefficient is analysed. The overall heat transfer coefficient increases with Dean number and its rate of increase at higher Dean number is low. The rate of heat transfer is maximum at a helix angle of 27°. Simulation studies show that the secondary flows that originate in a coiled tube due to curvature and torsion produce an additional transport of the fluid over the cross section of the pipe. The overall heat transfer coefficient increases with increase in inner diameter.

Numerical investigations on steam condensation in the presence of air on external surfaces of 3 × 3 tube bundles

Previous investigations on steam condensation in the presence of air have revealed general heat transfer characteristics in vertical plates and single tubes. However, there remains limited knowledge regarding the thermal-hydraulic phenomena and heat transfer properties of tube bundles. In order to evaluate steam condensation on the external surfaces of tube bundles, STAR-CCM + code with a diffusion boundary layer steam condensation model was employed in this work. In the assessments, 3 × 3 tube bundles with various tube pitches (from 1.5d to 5d) were investigated, and the results were compared to those of a single tube. The data analyses mainly focused on three aspects: (i) global and local condensation phenomena in tube bundles; (ii) heat transfer characteristics in tube bundles; and (iii) influences of tube pitches and global parameters (such as pressure and air mass fraction) on the condensation heat transfer. The phenomena analyses indicate that the axial field profiles are mainly determined by the development of a concentration boundary layer, while the radial field profiles are mostly affected by the tube pitches. Local heat flux analyses indicate that the condensation heat flux exhibits a significant reduction downwards along the vertical tube. In comparison, the condensation heat flux fluctuates in the tube peripheral direction. In the latter case, heat transfer enhancement and inhibition regions are identified and classified. For the tube average heat transfer, only the central tube is significantly affected by the tube pitches, and the degradation may be more than 30% at a tube pitch of 1.5d. The tube pitch effect is almost independent of the bulk air mass fraction and changes slightly with an increase in the wall sub-cooling and bulk pressure.

Numerical simulation and performance analysis of horizontal-tube falling-film evaporators in seawater desalination

A comprehensive distributed parameter model for simulating the steady-state performance of a practical horizontal-tube falling-film evaporator has been developed and validated. This model is capable of predicting the distributions of thermal parameters in the tube-side and shell-side, which provide important information of heat and mass exchange processes. The fluid flow and heat transfer characteristics in tubes are analyzed in detail. The computational time is reduced significantly in comparison with the Computational Fluid Dynamics. Based on the numerical results, it is found that the steam is not evenly distributed in the horizontal tubes of each tube pass, which is favorable for parallel channels with uneven heat fluxes. The mass and heat flux of steam are mutually matched, indicating that the self-compensation characteristic appears among the tubes. In addition, the overall heat transfer coefficient reaches the maximum value of about 3300 W/m 2 K at the entrance region of each tube pass, and then decreases gradually along the flow direction. As liquid film falls downward from tube to tube, the liquid flow rate of seawater continually decreases from 0.063 kg/ms to 0.04 kg/ms with the corresponding salinity gradually increasing from 36 g/kg to 56 g/kg.

Computational analysis of heat transfer and pressure drop performance for internally finned tubes with three different longitudinal wavy fins

Turbulent pressure drop and heat transfer characteristics in tubes with three different kinds of internally longitudinal fin patterns (interrupted wavy, sinusoidal wavy and plain) are numerically investigated for Re = 904–4,520. The channel velocity, temperature, and turbulence fields are obtained to discern the mechanisms of heat transfer enhancement. Numerical results indicate that the steady and spatially periodic growth and disruption of cross-sectional vortices occur near the tube/fin walls along the streamwise locations. The thermal boundary layers near the tube/fin surfaces are thereby periodically interrupted, with heat transfer near the recirculation zones being enhanced. The overall heat transfer coefficients in wavy channels are higher than those in a plain fin channel, while with larger pressure drop penalties. At the same waviness, the interrupted wavy fin tube could enhance heat transfer by 72–90%, with more than 2–4 times of pressure drop penalty. Among the fins studied, the sinusoidal wavy fin has the best comprehensive performance.

Closure to “Discussion of ‘Friction and Forced Convection Heat-Transfer Characteristics in Tubes With Twisted Tape Swirl Generators’” (1964, ASME J. Heat Transfer, 86, p. 48)

Closure to “Discussion of ‘Friction and Forced Convection Heat-Transfer Characteristics in Tubes With Twisted Tape Swirl Generators’” (1964, ASME J. Heat Transfer, 86, p. 48)

Friction and Forced Convection Heat-Transfer Characteristics in Tubes With Twisted Tape Swirl Generators

Velocity distributions, friction losses, and heat-transfer characteristics were studied analytically and experimentally for fully developed turbulent flow in tubes with twisted tape swirl generators. Data were obtained for pitch-to-diameter ratios from 3.62 to 22.0 with both air and water under isothermal and forced convection heating conditions. The following principal conclusions result: (a) The velocity field is helicoidal and corresponds to a forced vortex in the core superposed on an essentially uniform axial flow. (b) Friction losses may be predicted from the combined effects of the axial and tangential boundary-layer flows coupled with an additional “vortex-mixing” effect. (c) Approximate heat-transfer correlations can be predicted from a Colburn-type analysis. These may be improved by considering a boundary layer and vortex mixing model.

Discussion: “Friction and Forced Convection Heat-Transfer Characteristics in Tubes With Twisted Tape Swirl Generators” (Smithberg, E., and Landis, F., 1964, ASME J. Heat Transfer, 86, pp. 39–48)

Discussion: “Friction and Forced Convection Heat-Transfer Characteristics in Tubes With Twisted Tape Swirl Generators” (Smithberg, E., and Landis, F., 1964, ASME J. Heat Transfer, 86, pp. 39–48)