Methods of Heat Transfer Enhancement

            Twisted tape insertion in smooth plain tube is one type of passive methods for enhancing heat transfer. Swirl flow inside tube and associated heat transfer are very complex. In this paper numerical methods of computational fluid dynamics were performed with a commercially available CFD tool, ANSYS FLUENT (V 16.1) and ASPEN industrial program were used in analyzing and designing these heat transfer enhancement techniques. A circular plain tube has length L=8534mm and 17 mm inner diameter with twisted tape with a twist ratio of y = (H/D) = (150/17) =8.8 along with a plain tube were considered for this study. Eight Reynolds numbers (Re) of 784, 1000, 2000, 3000, 4000, 5000, 6000 and 7000 are considered to examine the sensitivity of thermal performance. Crude oil API 28 exit temperature, film heat transfer coefficient, Nusselt number and overall enhancement ratio results is presented for both empty and inserted plain tube with comparison between the two cases. An increase of 76% to 236% in overall enhancement is predicted with twist ratio 8.8 for Reynolds number 784 to 7000 respectively.

      Crude oil in order to be suitable for export, gases must be removed from it through special equipment and then need to be warming from 30°C to 70°C which is the suitable degree for de-salter process which was done in special equipment. Heated process of crude oil was done in tube bundle submerged in a large shell, full of water at 95°C to 98°C, where the water in shell heated by burners operated at the fuel gases that was separated and removed from the crude oil previously. Hundreds of tubes are used in tubes bundle. Therefore, it is economic to reduce number of tubes in the bundle with keeping the same heat transfer through enhancement heat transfer to the crude oil in tubes.

      Methods of heat transfer enhancement were used to increase the rate of heat transfer with keeping same overall system performance. Passive and active methods are employed as heat transfer enhancement techniques. Passive methods do not require an external power contrary to the active methods. Insertion of twisted tape is one of the most effective passive techniques. Twisted tapes produce swirl and turbulence, which are important for heat transfer enhancement. They also provide a longer flowing path and longer residence time, thus more efficient heat transfer and greater frictional loss. So that, in this study, inserted twisted tape was used to enhance the heat transfer coefficient inside tubes that flow in it crude oil API 28.

Eiams-aard, et al., (2006), studied the heat transfer and friction factor characteristics in a double pipe heat exchanger where the inner pipe was inserted with regularly spaced twisted tape elements. The heat transfer coefficient proportional directly with twist ratio, whereas the increase in the free space ratio, improves both the heat transfer coefficient and friction factor. Woei Chang, et al., (2007), studied experimentally the enhancement of heat transfer in a tube inserted with serrated twisted tape for different twist ratios. The augmentation of heat transfer with inserted serrated twisted tape was 1.25-1.67 times than with the plain twisted tape. Woei Chang et al., (2007), studied experimentally the heat transfer coefficients and axial pressure drop of the tube inserted with a broken twisted tape. Mean Fanning friction factors and local Nusselt numbers proportional inversely with the twist ratio. Bharadwaj, et al., (2009), experimentally compared the heat transfer and pressure drop characteristics of water flow in spirally grooved tubes and smooth tubes inserted with twisted tape. In the laminar range, spirally grooved tubes without twisted tapes yield more enhancement of heat transfer than the spirally grooved tubes with twisted tape inserts. Eiamsa-ard and Promvonge, (2010), experimentally studied arrangement effect of alternate clockwise and counterclockwise rotation of twisted tape inserts inside circular tube on heat transfer enhancement. Better heat transfer rate was observed than with typical twisted tape inserts. Naga Sarada et al., (2010), presented the results obtained of experimental investigations of turbulent flow heat transfer in a horizontal tube with varying width twisted tape inserts. The enhancement of heat transfers with twisted tape inserts varied from 36% to 48% as compared to plain tube for full width inserts. Thianpong, et al., (2011), investigated experimentally the effects of perforated twisted tape and plain twisted tape inside circular tube on heat transfer enhancement. It was found that tubes with perforated twisted tape and plain twisted tape yielded heat transfer enhancement up to 208% and 190% over plain tube, respectively. Murugesan, et al., (2011), investigated experimentally the effect of V-cut twisted tape inserted inside tube. Mean friction factor and mean Nusselt number increase with decreasing width ratios, twist ratios, and increasing depth ratios (ratio between depths of V cut to tape width). Bas and Ozceyhan, (2012), experimentally investigated the effect of clearance ratio (ratio between clearance and tube diameter) and twist ratio of twisted tape inserted inside tube on flow friction and heat transfer behavior. An increase in heat transfer rates with a reduction in clearance ratio was reported. Gunes, et al., (2012), investigated numerically the pressure drop and heat transfer in a tube fitted with regularly spaced twisted tape elements having different tape widths. Within a range of Reynolds number, the effect of space ratio and twisted tape width on the heat transfer and friction characteristics was reported. Giniyatullin and Tarasevich, (2013), investigated experimentally and numerically the effect of twisted tape inserted inside tube on heat transfer of subcooled flow boiling regime. The simulation clarified the influence of phase distribution due to centrifugal forces. It displayed that this force is dominant, especially when the vapor friction is high. The vapor accumulates in the angular zones when vapor friction has a small value. Because of different heat transfer rates of vapor and liquid, hot spots were occurring near the walls of those zones. Kumar Saha and Kumar Pai, (2014), studied experimentally Nusselt number and friction factor data for laminar flow of viscous oil inside a circular tube having integral spiral corrugation roughness and fitted with twisted tapes with oblique teeth. The twisted tapes with oblique teeth in combination with integral spiral corrugation roughness perform better enhancement technique than the individual techniques.

Ramakumar, et al , 2016, investigated numerically the performance of tapered twisted tapes (tape width decreases along the flow direction) inserted inside a circular plain tube for three Reynolds numbers of 8545, 11393, and 13333 with a twist ratio of 3 and taper angles of 0.3, 0.4, 0.5, 0.6, and 0.7 via CFD tool (ANSYS FLUENT v14.0). Heat transfer and pressure drop results were presented in the form of friction factor, Nusselt number, and overall enhancement ratio. An increase of 17% in overall enhancement was found with 0.5 taper angle over conventional tapes.

      From the fore mentioned studies, it was noticed that a good number of researchers focused their studies on heat transfer enhancement and fluid friction behaviors of circular tubes fitted with twisted tapes. A twisted tape enhances the heat transfer rates by fluid flow swirling caused by the centrifugal forces generated. Twisted tape helped with further enhancement of heat transfer rates, but with a considerable penalty on pressure loss. The present study aims at numerical analysis of proposed twisted tape with twisted ratio 8.8 inside plain tube for different Reynolds number. The operational performances of twisted tape are compared with empty plain tube.

2 PROBLEM FORMULATION AND NUMERICAL METHOD

     A three-dimensional steady system of laminar and turbulent flow in a plain tube has length L=8534mm and 17 mm inner diameter equipped with a twisted tape which has a twist ratio of y = (H/D) = (150/17) =8.8 along with a plain tube is considered in the present study. The physical model simulates the flow of crude oil in a heat exchanger tubes. Heat added to the outer tube walls via hot water was carried out by the crude oil from inner tube wall.

2.1 NUMERICAL SOLUTION

     In order to mathematically analyze the heat transfer characteristics of heat exchanger fluid flow in plain empty and inserted plain tube, a Navier-Stokes equations solution is required. Due to the complexity of twisted surface configuration and the significant viscous and heat effects, a numerical technique using the solver, ANSYS-FLUENT version 16.1 uses the finite-volume method to solve the governing equations through using a pressure-based solver.  Two turbulence model, and k-ε were used to solve governing partial differential equations of mass, momentum and energy conservations in three dimensions. A turbulence model involves solution of the four transport equations; turbulence kinetic energy, its rate of dissipation, velocity variance scale and elliptic relaxation function ƒ, to demonstrate the effect of the turbulence on the flow structure.

2.1.1 Mesh

      Pointwise V17.0R1 was used to generate structured, unstructured and hybrid grids. Structured grid was used for tube surface while unstructured grid was used for flow cross section which result a hybrid mesh in the block that a mixture from hexahedron, pyramid and prism cells as, shown in Fig. 2. A fine mesh has been created on the flow cross suction to resolve the thermal boundary layer.

     The solution to be accurate in this work, skewness kept up to 0.85 for hex, quad and triangular cells and up to 0.9 for tetragonal cells. For structured domains the orthogonality of grid points adjacent to the tube wall was kept to perfect orthogonality and max value 90° along the entire tube surface. For the present twisted model 5,769,257 cells and for empty plain tube model 4,941,189 cellswere used. The rate of convergence is indication to mesh quality. In this work the convergence was a chivied with about 500 iterations.

2.1.2 GOVERNING EQUATION AND ASSUMPTIONS

     The numerical study of this work was depended on the actual crude oil heat exchanger conditions that was specified and used in crude oil transport plant. Crude oil conditions represented by density 865 kg/m³, specific heat 1982J/kg. K, thermal conductivity 0.129 W/m.K and viscosity 0.00752kg/m. s at Reynolds number 784. Other values of Reynolds number were studied to predict the effect of Re on the thermal performance. In this research will focus on the effect of inserted twisted tape in plain tube on crude oil heat exchanger thermal performance for ranges of Reynolds number 784, 1000, 2000, 3000, 4000, 5000, 6000, and 7000 for both crude oil API28 flow.

     The characteristic Reynolds number depending on tube hydraulic diameter which equal to inner tube diameter as a characteristic length and 5% turbulent intensity was used at free stream inlet velocity.

     In the present study, crude oil API 28 is a fluid inside tube. The flow characteristics were assumed to be as follows:

1. Three-dimensional.
2. Incompressible flow, Ma<0.3.
3. Density varies only with temperature.
4. Laminar and turbulent flow.
5. Steady state flow in main flow.
6. Newtonian fluid.
7. Single-phase flow.
8. Flows pressure work  , and kinetic energy terms in energy equation are negligible.
9. Viscous dissipation terms are negligible.
10. 

2.1.3   TURBULENCE MODEL

      The flow inside tube is a boundary layer flow. The most suitable turbulent model for flow in side empty plain tube that considering the boundary layer is   model. The turbulence kinetic energy, rate of dissipation, the velocity variance scale, and the elliptic relaxation function can be obtained from the following transport equations:

     The model constants have the following default values:

2.1.4 TRANSPORT EQUATIONS FOR THE RNG k-ε MODEL

Mathematical technique called “renormalization group” (RNG) methods was used to model the  RNG-based k-e turbulence model that was derived from the instantaneous Navier-Stokes equations . RNG k-ε turbulence model with enhanced wall treatments was used to model the case study with effective accounts of viscosity for Low-Reynolds number. The effect of swirl on turbulence was included in the RNG model.

     The term G_k represents the generation of turbulence kinetic energy due to the mean velocity gradients. G_b is the generation of turbulence kinetic energy due to buoyancy. Y_M represents the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate. The quantities α_k and α_ε are the inverse effective Prandtl numbers for k and ε, respectively. S_k and S_ε are user-defined source terms. Hence, for the present study, the turbulent flow was modeled using the k-e viscous turbulence model with turbulent coefficients C=0.0845, C₁1.42, and C₂=1.68. Semi implicit pressure linked equation method (SIMPLE) was used for pressure velocity coupling.

     The second-order upwind discretization scheme is used for turbulence kinetic energy and dissipation. The second-order discretization scheme was used for momentum and energy. The convergence criterion of 10^-6 was chosen in addition to the selected monitored properties.

2.2 BOUNDARY CONDITIONS

      Boundary conditions were specified for each zone of the computation domain. For the steady state, there were three boundaries in the physical flow domain, inlet, outlet and solid surfaces wall as shown in Fig. 2. However, the internal domain zone that shares common areas faces does not require any boundary condition.

The boundary conditions that used in this work are as follow:

• Velocity-inlet boundary at, , , =17mm and pressure gage=0.
• Outflow boundary at, , =17mm.
• No-slip and constant tube wall temperature at 370K.
• No-slip and zero heat flux at the wall of twisted tape.
• Zero-gradient boundary condition for the variable at inlets with default value
• The direction of the flow was defined to be normal to the boundary.
• The wall of the tube was assumed to be perfectly smooth with zero roughness.

2.3 DATA REDUCTION

3 RESULTS AND DISCUSSION

      The use of twisted tape elements leads to a considerable increase in heat transfer and pressure drop over plain tube. Consequently, the numerical results reveal that the best operating regime of all twisted tape elements was found at low Reynolds number, leading to more compact heat exchanger. Validation was done with two turbulence models and a close value is obtained with the k-e turbulence model.

      The laminar model was considered for Reynolds number up to 2000 while v-2f and k-e turbulence models were used to solve the governing equations for Reynolds number above 2000. Figure 3 clarifybulk exit temperature of crude oil API 28 from empty plain tube and plain tube inserted with twisted tape. For plain empty tube, it was clear that the results of bulk exit temperatures that were calculated from ANSYS- Fluent and Aspen simulation program are very close, where only 0.3% to 0.9% temperature difference for Reynolds number up to 4000 respectively and 1.2% to 1.8% for Reynolds number 5000 to 7000 respectively.

      For plain tube inserted with twisted tape, the percentage difference in bulk exit temperature that were calculated from ANSYS-Fluent and Aspen simulation program were 3.7% to 0.29% for Reynolds number 784 to 7000 respectively, where the difference between two methods)ِ ANSYS-Fluent program and Aspen program) were less than 1% for turbulent flow (Re ≥ 3000) when using k-e turbulence model, while when using v-2f turbulence model the difference in bulk exit temperature that were calculated from ANSYS Fluent and Aspen simulation program were 0% to 3.6% for Re ≥ 3000 respectively. In the actual process, the Aspen simulation program was very close to actual data and its widely used from American manufacturing companies that were specialize in manufacturing processes equipment. Since the results from K-e turbulence model was very close to aspen simulation program than v-2f turbulence model, thus, K-e turbulence model was more accurate than v-2f model for simulated swirling flow inside tube, and the reason of that related to that the k-e model equation contain swirling flow parameter. While v-2f turbulence model was accurate for flow inside empty plain tube (flow without swirling) rather than K-e model which the solve diverging in it.

      It is clear from figure 4 that the static pressure difference between inlet and outlet boundary for plain tube inserted with twisted tape are higher than static pressure difference for empty plain tube. As shown in figure 5 the percentage increase of static pressure difference between inlet and outlet boundaries were 141%, 137%, 329%, 197%, 176%, 171%, 160% and 146% above the that for empty plain tube for Reynolds numbers 784, 1000, 2000, 3000, 4000, 5000, 6000 and 7000 respectively. It is clear that the higher static pressure difference percentage increase was happened at Re=2000, so it’s favorite to a void flow at Re=2000. The friction factor at inner surface tube wall for plain tube inserted with twisted tape was higher than of empty plain tube as shown in figure 6, which had the same percentage increases of figure 5 due to the relation between them related to equation 15.

      Film heat transfer coefficient of inner tube surface is approximately constant for laminar flow (Re ≤ 2000) where the percentage increase in film heat transfer coeff. due to inserted twisted tape was 1.9% to 7%. While the effect of twisted tape became sensible for turbulent flow where proportional directly with Reynolds number, where the percentage increase was 86% to 216% for Reynolds number 3000 to 7000 respectively, as shown in figures (7 & 8). Nusselt number take the same behavior and percentage of increasing of film heat transfer coefficient, regarding equation 14, as shown in figure 9. The overall enhancement ratio for laminar flow were approximately constant near 75%, while it arrived to 236% at Re=7000 as clear in figure 10.

4 CONCLUSIONS

The conclusions that were predicted from studding the effect of twisted tape inserted inside plain tube flow inside it crude oil API 28 enter at 313K on improvement the rate of heat transfer from tube wall which was keep at constant temperature 370K:

1. Significant effect of twisted tape insert in plain tube, on the heat transfer enhancement from tube surface, for turbulent flow. The overall enhancement ratio proportional directly with Reynolds number which arrived to 236% at Re=7000. Insensible effect of twisted tape insert for laminar flow where the overall enhancement ratio only 75%.
2. v-2f turbulence model is accurate and suitable model for simulation turbulent flow inside circular empty plain tube.
3. K-e turbulence model was more accurate than v-2f model for simulated swirling flow inside tube, and the reason of that related to that the k-e model equation contain swirling flow parameter.
4. The static pressure difference between inlet and outlet boundary for plain tube inserted with twisted tape are higher than static pressure difference for empty plain tube.
5. Film heat transfer coefficient of inner tube surface is approximately constant for laminar flow (Re ≤ 2000) where the percentage increase in film heat transfer coefficient due to inserted twisted tape was 1.9% to 7%. While the effect of twisted tape became sensible for turbulent flow where proportional directly with Reynolds number, where the percentage increase were 86% to 216% for Reynolds number 3000 to 7000 respectively,
6. The overall enhancement ratios for laminar flow were approximately constant near 75%, while it arrived to 236% at Re=7000.