Heat transfer enhancement through a square duct fitted with twisted tape inserts

Experimental investigations of friction factor and heat transfer characteristics of a square duct fitted with twisted tapes of different twist ratios have been reported at nearly uniform wall temperature conditions. The experimental results indicate that the friction factor and Nusselt number increases with decreasing twist ratio. The maximum heat transfer enhancement was observed for a minimum twist ratio. The thermohydraulic performance analysis is made to identify potential benefits of using a twisted tape.     

Experimental study of ethylene glycol-based Al2O3 nanofluid turbulent heat transfer enhancement in the corrugated tube with twisted tapes

In this study, fluid flow of the Al₂O₃/ethylene glycol (EG) nanofluid in a corrugated tube fitted with twisted tapes were experimentally studied under turbulent flow conditions. The experiments with different twists ratio and different nanofluid concentration were performed under similar operation condition. The investigated ranges are (1) three different Al₂O₃ concentrations: 0.5, 1 and 1.5 % by volume (2) three different twist ratios of twisted tape: y/w = 2, 3.6 and 5 and (3) Reynolds number from 6000 to 30,000. Regarding the experimental data, utilization of twists together with nanofluids tends to increase heat transfer and friction factor as compared with the base fluid. In addition, heat transfer performances were weakened by using for high nanoparticle concentration. The thermal performances of the heat exchanger with nanofluid and twisted tapes were evaluated for the assessment of overall improvement in thermal behavior. Over the range studied, the maximum thermal performance factor 4.2 is found with the use of Al₂O₃/EG nanofluid at concentration of 0.5 % by volume in corrugated tube together with twisted tape at twist ratio of 2.     

Heat transfer in turbulent tube flow inserted with loose-fit multi-channel twisted tapes as swirl generators

Heat transfer and flow behaviors in three-dimensional circular tubes with loose-fit multiple channel twisted tapes were numerically studied. The investigation was examined for Reynolds numbers(Re)ranging from 5000 to 15,000, by using air as testing fluid. Effects of the multiple channel number(N = 2, 3,and 4), clearance ratio(CR = 0.0, 0.025, 0.05, and 0.075) on heat transfer enhancement and flow friction were examined. The numerical results indicate that the tubes with loose-fit multiple channel twisted tapes perform higher heat transfer rates than the plain tube. The enhanced heat transfer rate is escorted with larger pressure drop. Both heat transfer and pressure drop increase with increasing multiple channel number(N) and decreasing clearance ratio(CR). Heat transfer augmented by the loose-fit multiple channel twisted tape with N = 4 is higher than those enhanced by the ones with N = 2 and 3 by around 9.5–17.8%and 5.8–7.8%, respectively. In addition, the loose-fit multiple channel twisted tapes with clearance ratio of 0.025, 0.05, and 0.075 give lower heat transfer rates than the one with CR = 0.0 by around 8.4%, 17.5%,and 28.8%, respectively.     

Further understanding of twisted tape effects as tube insert for heat transfer enhancement

Tube inserts are used as heat transfer enhancement tool for both retrofit and new design of shell and tube heat exchangers. This paper discusses and reviews the characteristics and performance of twisted tapes. The theory and application are also addressed. Industrial case study was selected to illustrate the behaviour effect that the twisted tapes impose at various laminar, transition and turbulent flow regions. This effect was demonstrated by changing the inside tube diameter and twist ratio through evaluating selected exchanger design parameters such as: local heat transfer coefficient, friction factor and pressure drop. Testing the exponent powers for Re and Pr at both laminar and turbulent regions were carried out. General design considerations are outlined for the use of twisted tapes in shell and tube heat exchangers.     

Numerical prediction of laminar flow and heat transfer characteristics in a tube fitted with regularly spaced twisted-tape elements

By numerically solving the Navier-Stokes and energy equations in their three-dimensional parabolic form, the friction and heat transfer characteristics are predicted for laminar flow in a circular tube fitted with regularly spaced twisted-tape elements that are connected by circular rods. The predictions have agreed closely with the experimental data for water. It is shown that a strong L/D effect is associated with the configuration under consideration. By extending the predictions outside the range of experimental data, it is found that considerably enhanced thermohydraulic performance is achievable by increasing the number of turns on the tape elements, by reducing the connecting rod diameter, and at high fluid Prandtl numbers.     

Studies on heat transfer in flow of silver nanofluid through a straight tube with twisted tape inserts

Convective heat transfer in the flow of silver nanofluid through a straight tube with twisted tape inserts was investigated experimentally. This straight tube was used as absorber/receiver tube in parabolic trough collector. The experiments were conducted for Reynolds number range 500 < Re < 6000 with twisted tape inserts of different twist ratio range 0.577 < H/D < 1.732. This experimental study shows that twisted tape inserts enhances heat transfer rate in the tube. The heat transfer coefficient and friction factor in the flow of silver nanofliud with 5 % volume fraction (concentration) are higher compared to the flow of water. From this study, Nusselt number, friction factor and enhancement factor are found as 2.0–3.0 times, 10–48.5 and 135–175 %, respectively with silver nanofliud. Finally new possible correlations for predicting heat transfer and friction factor in the flow of silver nanofliud through the straight tube with twisted tape inserts are proposed.     

Turbulent heat transfer for pipe flow with uniform heat generation

An Analytic solution is presented of the problem of turbulent heat transfer in pipes with internal heat generation and insulated wall by applying a recently-developed eddy conductivity model. The results agree closely with available experimental data for a wide range of Prandtl number (0.02–10.5).     

3D Numerical heat transfer and fluid flow analysis in plate-fin and tube heat exchangers with electrohydrodynamic enhancement

Three-dimensional laminar fluid flow and heat transfer over a four-row plate-fin and tube heat exchanger with electrohydrodynamic (EHD) wire electrodes are studied numerically. The effects of different electrode arrangements (square and diagonal), tube pitch arrangements (in-line and staggered) and applied voltage (V_E=0–16 kV) are investigated in detail for the Reynolds number range (based on the fin spacing and frontal velocity) ranging from 100 to 1,000. It is found that the EHD enhancement is more effective for lower Re and higher applied voltage. The case of staggered tube pitch with square wire electrode arrangement gives the best heat transfer augmentation. For V_E=16 kV and Re = 100, this study identifies a maximum improvement of 218% in the average Nusselt number and a reduction in fin area of 56% as compared that without EHD enhancement.     

Turbulent heat transfer in circular tube flows of viscoelastic fluids

The effects of thermal entrance length, polymer degradation and solvent chemistry were found to be critically important in the determination of the drag and heat transfer behavior of viscoelastic fluids in turbulent pipe flow. The minimum heat transfer asymptotic values in the thermally developing and in the fully developed regions were experimentally determined for relatively high concentration solutions of heat transfer resulting in the following correlations: $$\begin{gathered} j_H = 0.13\left( {\frac{x}{d}} \right)^{ - 0.24} \operatorname{Re} _a^{ - 0.45} thermally developing region \hfill \\ x/d< 450 \hfill \\ j_H = 0.03 \operatorname{Re} _a^{ - 0.45} thermally developed region \hfill \\ x/d< 450 \hfill \\ \end{gathered} $$ For dilute polymer solutions the heat transfer is a function ofx/d, the Reynolds number and the polymer concentration. The Reynolds analogy between momentum and heat transfer which has been widely used in the literature for Newtonian fluids is found not to apply in the case of drag-reducing viscoelastic fluids.     

Turbulent flow and convective heat transfer in a wavy wall channel

This paper reports the numerical modeling of turbulent flow and convective heat transfer over a wavy wall using a two equations eddy viscosity turbulence model. The wall boundary conditions were applied by using a new zonal modeling strategy based on DNS data and combining the standard k– turbulence model in the outer core flow with a one equation model to resolve the near-wall region.It was found that the two-layer model is successful in capturing most of the important physical features of a turbulent flow over a wavy wall with reasonable amount of memory storage and computer time. The predicted results show the shortcomings of the standard law of the wall for predicting such type of flows and consequently suggest that direct integrations to the wall must be used instead. Moreover, Comparison of the predicted results of a wavy wall with that of a straight channel, indicates that the averaged Nusselt number increases until a critical value is reached where the amplitude wave is increased. However, this heat transfer enhancement is accompanied by an increase in the pressure drop.     

Turbulent heat transfer characteristics of water flow in a rotating pipe

Large-Eddy-Simulation of turbulent heat transfer for water flow in rotating pipe is performed, for various rotation ratios (0 ≤ N ≤ 14). The value of the Reynolds number, based on the bulk velocity and pipe diameter, is Re = 5,500. The aim of this study is to examine the effect of the rotating pipe on the turbulent heat transfer for water flow, as well as the reliability of the LES approach for predicting turbulent heat transfer in water flow. Some predictions for the case of non-rotating pipe are compared to the available results of literature for validation. To depict the influence of the rotation ratio on turbulent heat transfer, many statistical quantities are analyzed (distributions of mean temperature, rms of fluctuating temperature, turbulent heat fluxes, higher-order statistics). Some contours of instantaneous temperature fluctuations are examined.     

Heat transfer and thermal performance characteristics of heat exchanger tube fitted with perforated twisted-tapes

Twisted tape insert was applied as a swirling flow generator for the passive heat transfer enhancement in the present work. The influences of the perforated twisted tapes (PTs) on the heat transfer, pressure loss and thermal performance characteristics were investigated experimentally. The experiments were performed under uniform wall heat flux condition by using PTs with y/W?=?3, 4 and 5, d/W?=?0.11, 0.14 and 0.17 and s/W?=?0.4, 0.6 and 0.8 where y is a twist length, d is a perforation hole diameter, s is a spacing between holes (pitch) and W is a tape width. The experimental results reveal that Nusselt number increased with decreasing s/W and y/W and increasing d/W. For the present range, the maximum heat transfer was obtained by utilizing the tape with s/W?=?0.4, d/W?=?0.17 and y/W?=?3, which is higher than those obtained from the plain tube with and without typical twisted tape by around 27.4 and 86.7%, respectively. In addition, the empirical correlations for Nusselt number, friction factor and thermal performance are also proposed in the present paper.     

A computational work on turbulent flow and heat transfer in a channel fitted with inclined baffles

Numerical studies of momentum and heat transfer characteristics have been investigated of a steady incompressible turbulent flow of air through channel. The channel has inclined baffles which are arranged on the walls in a periodically staggered way. The governing equations, namely, continuity, Navier–Stokes and energy, based on k–ω turbulence model to describe the turbulence phenomenon are solved using the finite volume method and the SIMPLE algorithm. Calculations are performed for a Reynolds number between 12,000 and 38,000. The axial velocity profiles, the velocity fields, the local and average coefficient of friction and the Nusselt number distribution were obtained for all the geometry considered and for different sections selected, upstream, downstream and between the two inclined baffles. Simulation results which were obtained by the use of baffles are validated by an experimental study. Good agreement is observed between numerical and experimental results data in the literature.     

The relationship between absolute vorticity flux along the main flow and convection heat transfer in a tube inserting a twisted tape

As passive enhancement devices, twisted tape insert has been used for almost a century, the most dominant heat transfer enhancement mechanism of circular tube fitted with twisted tape is the secondary flow generated by the tape. There is a parameter to specify the intensity of secondary flow, but this parameter cannot be applied to more general cases. Here cross-averaged absolute vorticity flux in the main flow direction is used to specify the intensity of secondary flow produced by twisted tape inserted in a tube. The relationship between the intensity of secondary flow and the intensity of laminar convective heat transfer is studied using a numerical method. The results reveal that the cross-averaged absolute vorticity flux in the main flow direction can reflect the intensity of secondary flow and a significant relationship between this cross-averaged absolute vorticity flux and Nusselt number exists for studied cases. The presented results validate that the cross-averaged absolute vorticity flux in the main flow direction is a general specifying of the intensity of secondary flow and can be used in other cases.     

Turbulent heat transfer in a flow of liquid metal near the wall

The article discusses turbulent heat transfer in media with small Prandtl numbers (Pr?1 for liquid metals). In this case, the thermal sublayer is Pr^-1 times thicker than the viscous sublayer. It is established that the coefficient of turbulent heat transfer varies in the thermal sublayer proportionally to the second power of the distance to the wall; the ratio of the coefficients of the turbulent transfer of heat and momentum in this region decreases in accordance with a linear law with approach to the wall. The conclusions of the theory are compared with the experimental data of other authors.     

Augmentation of heat transfer by twisted tape inserts during condensation of R-134a inside a horizontal tube

An experimental investigation has been carried out to find the heat transfer coefficient during condensation of R-134a vapor inside a horizontal tube. Experiments were conducted for the condensation of R-134a inside a plain tube and tubes with different twisted tape inserts. Twisted tapes with different twisted ratios of 6, 9, 12 and 15 were inserted in the refrigerant side, one by one, in the full length of test-condenser. For each inserted tube and the plain tube, test runs were carried out for the mass velocities of 92, 110, 128 and 147 kg/s-m². An empirical correlation has also been developed to predict the enhanced heat transfer coefficient.     

Turbulent transient heat transfer in channel flow with step change in inlet temperature

The transient forced convection in tubulent channel flow with a step change in inlet temperature is solved by using a hybrid scheme, namely, by combining the generalized integral transform technique with a second-order accurate finite-differences. Stability and convergence of the scheme are examined. Numerical results are presented for the fluid bulk temperature and local Nusselt number as a function of position along the channel at different times, and the propagation of the thermal wave front during temperature transients is examined.

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Nichtstationärer turbulenter Wärmeübergang bei Kanalströmung mit sprunghafter Änderung der Eintrittstemperatur

Zusammenfassung Der nichtstationäre Wärmeübergang bei turbulenter erzwungener Konvektionsströmung in Kanälen nach sprunghafter Änderung der Eintrittstemperatur wird mit Hilfe eines Hybridmodells untersucht. Dieses kombiniert die verallgemeinerte Integraltransformationstechnik mit einem Finite-Differenzen-Schema zweiter Ordnung von hoher Genauigkeit. Stabilität und Konvergenz des Verfahrens werden untersucht und numerische Ergebnisse für die Mischtemperatur und die lokale Nußelt-Zahl mitgeteilt und zwar als Funktion von Zeit und Kanallängskoordinate. Auch die Ausbreitung der thermischen Störungsfront nach Temperatursprung wird untersucht.

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Nomenclature A _ik coefficients matrix, Eq. (6) - A* _ik coefficients matrix, Eq. (13 b) - c _i eigenvalues of linearized coefficients matrixA* - C maximum to mean velocity ratio - D _e equivalent diameter (= 4r _w ) - E(R) dimesionless total diffusivity (= 1 + _h /) - N number of terms in eigenvalue expansion - N _i normalization integral, Eq. (3 c) - NT,NZ total number of nodes in andZ variables - Nu(Z) local Nusselt number - Pr Prandtl number (=/) - Pe Peclet number (=Re Pr) - Re Reynolds number (=u _m De/) - r _w channel radius - r transverse coordinate (dimensional) - R transverse coordinate (dimensionless) (=r/r _w ) - t time (dimensional) - T ₀,T _i initial and inlet temperature, respectively (dimensional) - T (r,z,t) fluid temperature (dimensional) - u _m mean flow velocity - u _max maximum flow velocity - u(r) velocity distribution (dimensional) - W (R) velocity distribution (dimensionless) (=u(r)/Cu _m ) - z axial coordinate (dimensional) - Z axial coordinate (dimensionless) (= 16z/C Re Pr D _e ) Greek letters thermal diffusivity of the fluid - Courant number - _h eddy diffusivity for heat - _m eddy diffusivity for momentum - parameter of discretization, Eq. (8c) - (R, Z, ) dimensionless temperature (= [T (r, z, t) –T ₀/T _i –T ₀]) - _av (Z, ) dimensionless fluid bulk temperature, Eq. (10) - ( _i ,R) eigenfunctions of Sturm-Liouville problem (2) - _i eigenvalues of Sturm-Liouville problem (2) - dimensionless time (=t/r ₂ ^w ) - kinematic viscosity