CFD Simulation of Heanched Heat Transfer in Concentric Tube with Inner Twisted Tape Insert By Using Nano Fluids

CFD Simulation of Heanched Heat Transfer in Concentric Tube with Inner Twisted Tape Insert By Using Nano Fluids

1J. Balabhaskara Rao, 2D. Bhanuchandrarao, M. Ashok chakravarthy3, K. Srinivasarao4 1Associate Professor, Mechanical Engineering Department, Vaishnavi Engg.College, Srikakulam1 2M.Tech Student, Thermal engineering, V.R Siddhartha Engineering College, JNTU

3Assistant Professor, Mechanical Engineering, V.R Siddhartha Engineering College, JNTU

4Asst. Professor, Mechanical Engineering Department, Vaishnavi Engg.College, Srikakulam4

Abstract Heat transfer for heating and cooling fluids play an important role in many industries. The past literature reporting heat transfer enhancement techniques in different processes are abundant. Most of these enhancing techniques are based on flow geometry parameters (like fins arrangement or inserts) and/or improving the thermal properties of fluids. Conventional heat transfer fluids such as water, ethylene glycol (EG), propylene glycol (PG) and engine oil have inherently low thermal properties relative to nanofluids such as CuO, Al2O3 and TiO2 fluids. Further increment the heat transfer rate of plane tube by inserting the twisted tape.

The present project is CFD simulation analysis for thermal-hydraulic characteristics of CuO+(30%PG+70%water) different concentration (0.025% and 0.1% CuO) nanofluids flow inside a circular tube and with inserts. For the heat transfer in a circular tube fitted with twisted tape inserts, the parameters are pitch to inlet diameter ratio H/D=10 and H/D=5 spacer in laminar and turbulence flow conditions has been explained in this thesis using ansys fluent 6.2.3 taken up for the estimation of heat transfer enhancement of nanofluids at different volume concentrations under laminar and turbulent flow Reynolds number at constant heat flux boundary condition.

1 INTRODUCTION

Heat transfer enhancement is the process of improving the performance of a heat transfer system by increasing the heat transfer coefficient. In the past decades, heat transfer enhancement technology has been developed and widely applied to heat exchanger applications; for example, refrigeration, automotives, process industry, chemical industry etc. The performance of conventional Heat transfer fluids such as water, oils, ethylene glycol and propylene glycol are often limited by their low thermal conductivities.

Overcome these limitations some techniques have been developed for heat transfer enhancements over the years either to accommodate high heat fluxes in the limited area available or to reduce the size and consequently the cost in order to compete in the global market. Heat transfer enhancement is associated using external energy on the fluid through forced

flow vibration / jet impingement and the use of electrostatic fields.

2 LITERATURE SURVEY

2.1 MAJOR CONTRIBUTORS

A number of experimental studies have been performed to investigate the transport properties of nanofluids, Eastman et al.(1997), Wang et al.(1999), Lee et al.(1999) and Xuan and Li (2000). These studies are concentrated on the evaluation of effective thermal conductivity under macroscopically stationary conditions. Limited studies on other aspects related to nanofluids such as phase change behavior has been due to Das et al.(2003), Tsai et al.(2003), You et al.(2003) and Vassallo et al.(2004).

Heris et al. [2006] conducted experiments in the laminar range with Al2O3 and CuO nanofluids and observed Al2O3 nanofluid to have higher heat transfer rates compared to CuO nanofluids.

Dittus Boelter [1930], Gnielinski [1976], Tam and Ghajar [2006] and Churchill and Usagi [1972] developed correlations for estimation of heat transfer coefficient of single-phase fluid flow in a circular tube under fully developed and transitional flow conditions under constant heat flux boundary condition.

Xuan and Li [2003] estimated convective heat transfer coefficient of water+Cu nanofluid and found substantial heat transfer enhancement. At a Reynolds number of 20,000, the heat transfer coefficient containing 2% volume concentration of Cu nanoparticles in water is shown to be approximately 60% higher than that of pure water.

Single phase heat transfer enhancement with twisted tape inserts under laminar and turbulent flow conditions have been dealt by Smithberg and Landis [1964], Sarma et al. [2002] and with longitudinal strip inserts by Heish and Huang (2000), Saha and Langille [2002]. Experiments with nanofluids for heat transfer enhancements using twisted tape

inserts or longitudinal tapes have not been reported in literature.

Maiga et al. (2005) in this paper, we have investigated, by numerical simulation the hydrodynamic and thermal characteristics of a laminar forced convection flow of nanofluids inside (a) a straight heated tube and (b) a radial space between coaxial and heated disks. Two particular nanofluids were considered, namely Ethylene GlycolcAl2O3 and watercAl2O3. Results have clearly revealed that the addition of nanoparticles has produced a remarkable increase of the heat transfer with respect to that of the base liquids.

Sarma et al. [2008] have developed a correlation for the estimation of eddy diffusivity; Nusselt number of Al2O3 and CuO nanofluid in turbulent flow based on the experimental data published by various authors.

Bergman [2009] have been investigated the convective heat transfer coefficient of nanofluid under laminar flow condition and found that obtained specific heat of nanofluid is less compared to the specific heat of base fluid under same flow condition.

Sarma et al. [2002] making use of the equation of Smithberg and Landis [1964] for friction factor for flow in tubes under swirl flow observed that the classical equation of Van Driest for eddy diffusivity can be used successfully, if the coefficient in the equation is considered as a function of Reynolds number and twist ratio. They showed the variation of universal constant K for different twist ratios. Sarma et al. [2003] extended the analysis in the laminar flow range using the data of Kishore [2001] for friction factor obtained with Turbinol, thus encompassing a wide range of Reynolds and Prandtl number and also theoretically analyzed the convective heat transfer and friction factor of single phase fluid in plain tube with twisted tape inserts under laminar flow condition.

Sarma et al. [2005] theoretically developed a combined approach to predict the heat transfer and friction factor of single phase fluid in plain tube with twisted tape inserts at wide range of Reynolds number and Prandtl number.

Kumar and Prasad [2000] investigated the heat transfer and friction factor of water in solar water heating system with twisted tape inserts and found appreciable enhancement with inserts.

Klaczak [2001] presented experimental values of heat transfer enhancements with twisted tape insert in both in vertical and horizontal pipe at low Reynolds number range.

Saha and Dutta [2001] have been studied heat transfer and friction factor in circular tube by considering the uniform pitch, varying pitch and spacing between the pitch of

nanofluid in laminar to turbulent flow conditions in plain tube with and without inserts.

Heat transfer and friction factor of nanofluid in circular tube at low volume concentrations under the Reynolds number ranging from 500<Re<50000 at constant heat flux condition is estimated by using cfd simulation and the results are compared with the available literature and theory is developed.

The heat transfer coefficient and friction factor of diffrent volume concentration of nanofluid and with twisted and insert (Twist ratios, H/D = 5 and10)..

2.3 PRESENT INVESTIGATION

CFD simulation analysis and Estimation of thermo- physical properties in addition of additives of CuO nano particle in base fluid in the volume concentrations of 0.025% and 0.1% is taken and heat transfer coefficient and friction factor of CuO nanofluid at Reynolds number range of 500 to 50,000 subjected to constant heat flux boundary condition. Heat transfer enhancements are Estimation with displaced enhancement devices like twisted tape inserts in tube at that Reynolds numbers.

3 ESTIMATE THE PROPERTIES OF NANOFLUID

1. INTRODUCTION

   Thermo physical properties of nanofluid are essential for the evaluation of heat transfer coefficient. These roperties vary with concentration and temperature. The effect of variation of these parameters on thermo physical properties has been dealt by Lee et al. [1998], Das et al. [2000], Xuan and Roetzel [2003], Pak and Cho [1998] and Choi et al. [2003].

   Thermo physical properties of base fluid (30% propylene glycol + water) are taken form ASHRAE HAND BOOK [1972]. These properties are given in table 3.1.

   Table 3.1 Thermo-physical properties of base fluid from ASHRAE hand book.

2. VOLUME CONCENTRATION ESTIMATION

The quantity of copper oxide Wa to be added for different volume concentrations can be estimated from the relation

Wa

twisted tape inserts.

% volume concentration = a

2.2 SCOPE OF THE PRESENT WORK

It is observed from the literature that the experimental data for different nanofluids with twisted tape insert in laminar and turbulent flow conditions are available. However, no cfd simulation results are available for different volume

Where

a

Wa Ww

a w

Density of alumina, 6310 kg/m3

concentrations of nanofluid in laminar to turbulent flow conditions with and without twisted inserts. The present aim is

w Density of water, kg/m3

to estimate the heat transfer coefficient and friction factor of Ww

Weight of sample water says 100 grams

1. EVALUATION OF PROPERTIES

   The thermo-physical properties of nanofluid like

   Table 3.3 shows the viscosity of CuO nanofluid with values of Einstein correlation

   density, thermal conductivity, absolute viscosity, and specific heat are estimated with equations available in literature.

   1. Density

      Density of different volume concentrations of nanofluid is estimated by using Hydrometer and compared with the values of Pak and Cho [1998]. Density at different temperatures is estimated by Pak and Cho [1998] has given

      a regression formula.

      Thermal

      Specific

      nf

      p

      1 w

      Temperature	Density	conductivity	Viscosity	heat
      (ok)	(kg/m3 )	(w/m.k)	(mPa-s)	(j/kg.k)
      273	1036.24	0.409	7.07	3.654548
      283	1032.55	0.421	4.52	3.655633
      293	1028.35	0.431	3.06	3.662165
      303	1023.64	0.441	2.19	3.673319
      313	1018.42	0.45	1.63	3.688373
      323	1012.69	0.457	1.26	3.706677
      333	1006.44	0.463	1.01	3.727682
      343	99.69	0.469	0.83	3.750913
      353	992.42	0.473	0.7	3.775986
      363	984.65	0.476	0.6	3.802601
      373	976.36	0.478	0.53	3.830547

      Temperature	Density	conductivity	Viscosity	heat
      (ok)	(kg/m3 )	(w/m.k)	(mPa-s)	(j/kg.k)
      273	1036.24	0.409	7.07	3.654548
      283	1032.55	0.421	4.52	3.655633
      293	1028.35	0.431	3.06	3.662165
      303	1023.64	0.441	2.19	3.673319
      313	1018.42	0.45	1.63	3.688373
      323	1012.69	0.457	1.26	3.706677
      333	1006.44	0.463	1.01	3.727682
      343	99.69	0.469	0.83	3.750913
      353	992.42	0.473	0.7	3.775986
      363	984.65	0.476	0.6	3.802601
      373	976.36	0.478	0.53	3.830547

      [3.2]

      Table: 3.1 Comparison of density with Pak and Cho [1998]

      Sl.No.	Volume fraction, , (%)	Pak and Cho [1998]
      1	0.025	1027
      2	0.1	1031
      3	0.4	1047
      4	0.8	1068
      5	1.2	1089

   2. Thermal conductivity

Wasp [1977] proposed a thermal conductivity correlation in terms of nanoparticle volume concentration in the base fluid, which is given by

Kp 2Kw 2 Kw Kp

Knf Kw Kp 2Kw Kw Kp

Where, Kp, Knf , Kw are the thermal conductivity

coefficients for the dispersed phase, mixture phase and continuous phase of the fluid and is volume concentration

3.3.4 Specific heat

The specific heat of nanofluid is an important parameter that has to be valuated theoretically. In this analysis nanofluid (nanoparticles + base fluid) is considered as homogeneous mixture. This formula is applied for the estimation of specific heat of nanofluid at different volume concentrations. The specific heat formula for homogeneous mixture is given by

in percentage. Kc is the thermal conductivity of the base C

(m * Cp)CuO (m * Cp)bf

fluid at some reference temperature.

Table 3.2 Thermal conductivity of CuO nanofluid

p nf

mCuO

• mbf

from wasp equation in literature

3.3.3 Absolute viscosity

Einstein [14] has proposed a viscosity correlation in terms of nanoparticle concentration in the base fluid, when the nanoparticle volume concentration is lower than 5%, which is given by

The Cp of CuO is 760 J/kg K

1 5

(3.4)

s bf 2

The Einstein correlation is used to absolute viscosity calculation at different temperatures.

Table 3.4 shows the specific heat of CuO the values of homogeneous mixture formulae

heat

All the thermo physical properties of different volume concentrations of nanofluid and at different temperature are estimated from the theoretical correlation are shown in the Table 3.6.

Table 3.5 thermo-physical properties different volume concentration nanofluid

Table 3.6 thermo-physical properties 0.025% volume concentration nanofluid at different temperature

Table 3.7 thermo-physical properties 0.1% volume concentration nanofluid at different temperature

3.4 FINITE VOLUME METHOD

The mass, momentum, and scalar transport equations are integrated over all the fluid elements in a computational domain using CFD. The finite volume method is a particular finite differencing numerical technique, and is the most common method for calculating flow in CFD codes. This section describes the basic procedures involved in finite volume calculations.

The finite volume method involves first creating a system of algebraic equations through the process of discretising the governing equations for mass, momentum, and scalar transport. To account for flow fluctuations due to turbulence in this project, the RANS equations are discretised instead when the cases are run using the k-epsilon turbulence model. When the equations have been discretised using the appropriate differencing scheme for expressing the differential expressions in the integral equation (i.e. central, upwind, hybrid, or power-law, or other higher-order differencing schemes), the resulting algebraic equations are solved at each node of each cell.

3.6 SOLUTION TECHNIQUE

The value of the scalar properties of interest (i.e. temperature) at a particular location in the computational domain depends on the flows direction and velocity, which must also be solved for in the calculation process. To There are many algorithms available for this purpose, the most popular are the SIMPLE and PISO methods. This section describes the SIMPLE algorithm and compares it to the PISO algorithm. In this project SIMPLE algorithm is used in calculation process.

PROBLEM DESCRIPTION AND MODELLING

1. Cfd Geometry Modeling

   The geometry was making in catia (the CAD program). This	Nanofluid at 0.025%CuO concentration:	2.1914	mPa-
   geometry imported to ansys icem cfd and repairs the geometry.	sNanofluid at 0.025%CuO concentration:	2.1955	mPa-s
   The geometry consist of a cylindrical tube of diameter 14.5	Fluid thermal conductivityBase fluid:	0.441	W/mK
   mm diameter and length of 2000 mm. The plain tube with	Nanofluid at 0.025%CuO concentration:	0.4413	W/mK
   helical twist configuration consists of a plain tube with above	Nanofluid at 0.025%CuO concentration:	0.4423	W/mK
   diameter and length, and a helical twist with twist ratio of 5	Fluid densityBase fluid :	1023.6	Kg/m3
   and 15 its lengths of 1700mm as shown in fig 5.1	Nanofluid at 0.025%CuO concentration:	1025.3	Kg/m3
   	Nanofluid at 0.025%CuO concentration:	1028.9	Kg/m3

   The geometry was making in catia (the CAD program). This	Nanofluid at 0.025%CuO concentration:	2.1914	mPa-
   geometry imported to ansys icem cfd and repairs the geometry.	sNanofluid at 0.025%CuO concentration:	2.1955	mPa-s
   The geometry consist of a cylindrical tube of diameter 14.5	Fluid thermal conductivityBase fluid:	0.441	W/mK
   mm diameter and length of 2000 mm. The plain tube with	Nanofluid at 0.025%CuO concentration:	0.4413	W/mK
   helical twist configuration consists of a plain tube with above	Nanofluid at 0.025%CuO concentration:	0.4423	W/mK
   diameter and length, and a helical twist with twist ratio of 5	Fluid densityBase fluid :	1023.6	Kg/m3
   and 15 its lengths of 1700mm as shown in fig 5.1	Nanofluid at 0.025%CuO concentration:	1025.3	Kg/m3
   	Nanofluid at 0.025%CuO concentration:	1028.9	Kg/m3

   Fig 5.1 geometrical model of plane tube with twisted tape inserts

2. Grid Generation

   The mesh has been generated in the Cartesian co-ordinate system. The grid independence test has been conducted and the grid has been refined in tube wall region. The generated grids are shown in Fig. 5.2.

   The total number of cells in computational domain is 2, 12,480.

   Fig 5.2. Grid for the twisted tape insert

3. Modeling parameters or Boundary Conditions

Boundary conditions are used in the simulations are as shown in fig 5.3 and different inlet velocities are shown in table 5.1

Table 5.1 Numerical values of the parameters used for simulation

Boundary Conditions of tube with twisted tape insert

Pressure outlet (101325Pa)

Fluids specific heat Base fluid: 3.6733 KW/KgK Nanofluid at 0.025%CuO concentration: 3.6724KW/KgK Nanofluid at 0.025%CuO concentration: 3.6698KW/KgK

1. Relaxation

   The following relaxation factors for the various variables have been considered:

   P1 – pressure, linear relaxation (LINRLX) : 1.0

   U1 – velocity vector in X-axis, false-time-step relaxation (FALSDT) : 0.1

   V – velocity vector in Y-axis, false-time-step relaxation (FALSDT) : 0.1

   W1- velocity vector in Z-axis, false-time-step relaxation (FALSDT) : 0.1

   TEM1 – Temperature, false-time-step relaxation (FALSDT)

   : 1.0

   The relaxation parameters of other variables have been kept as the default values.

2. Convergence

The convergence criteria considered in the present investigation included stabilization of variables at different monitoring points as well as the mass & energy balance. The number of iterations needed to get the convergence was 1000 and/or convergence factor is taken as 0.001.

5.6 Numerical procedure and computational methodology

The governing differential transport equations were converted to algebraic equations before being solved numerically. After the specification of the boundary condition, the solution control and the initialization of the solution have to be given before the iteration starts. The solution controls like the pressure velocity coupling and the discrimination of the different variables and the relaxation factors have to be

specified. The solutions sequential algorithm (called the

Velocity inlet boundary condition (Based on Reynolds Number)

Entrance and exit outer walls : adiabatic

Tube outer W all: Heat flux boundary condition (8000w/m2)

Copper tube thickness

segregated solver) used in the numerical computation requires less memory that the coupled solver. Since we are using the segregated solver for our problem, the default under relaxation factors are used and the SIMPLE scheme for the pressure velocity coupling is used and the second discrimination is

Tube inner W all: coupled

Twisted tape : Solid

Fluid : Nanofluid

used for the momentum and the standard scheme is used for the pressure.

Fig 5.3 Boundary conditions of plane tube with twisted inserts Heat flux : 8340 W/m2

Pressure outlet : 101325 Pa

Inlet tempter : 303 K

Fluid viscosity Base fluid : 2.19 mPa-s

RESULT AND DISCURSIONS

The observations of CFD simulation results are discussed below for the given boundary conditions.

1. variation of pressure drop in plain tube with and with out twisted tape

   Fig. 6.1 shows that the pressure drop in a plain tube with and without twisted insert along its length. By referring y to the fig 6.1 one can observe that the pressure drop increases along the length with decreasing the twist ratio and in table 6.1, 6.2 and 6.3 can observe that the pressure drop increases with volume concentration of nano particles.

   Fig 6.1 variation of pressure drop in plain tube with and with out twisted tape

2. Velocity distribution in plain tube with and with out twisted tape

   Fig. 6.2 shows fluid flow pattern in a plain tube along its length. At the inlet of constant low temperature Nanofluid, the velocity of Nanofluid varies in the length direction. By referring to the Fig.1 one can observe that the inlet section the velocity profile can be change along the length unto sustain distance is called entrance length and remaining length the velocity profile doesnt change.

   Fig. 6.3 and 6.4 shows that the fluids flow pattern in a plain tube with twisted insert along its length. By referring to the Fig.6.2 and 6.3 one can observe at the middle section of the velocity path lines the velocity is more at one side of twist and also observed that the maximum velocity of fluid flow decreases with increasing the twist ratio.

   1. At middle of plain tube

      b) At the inlet of plain tube

      1. at the outlet of plain tube

         Fig 6.2 velocity magnitude and path lines of plane tube along the length at different sections

         Inlet Side Middle Outlet Side

         Inlet Side Middle Outlet Side

         1. At middle of plain tube

         2. At the inlet of plain tube

         3. at the outlet of plain tube

            Fig 6.3 velocity magnitude and path lines of plane tube with insert H/D=5 along the length at different sections

            Inlet Side Middle Outlet Side

            1. At middle of plain tube

            2. At the inlet of plain tube

      (d) at the outlet of plain tube

      Fig 6.4 velocity magnitude and path lines of plane tube with insert H/D=10 along the length at different sections

3. friction factor and heat transfer enhancement in Plain tube

   The results obtained in this simulation study are presented and discussed in this section. Figures show the variation of friction factor with Reynolds number for plain tube and the variation of heat transfer coefficient with Reynolds number for plain tube. By referring to the Figures one can observe that as Reynolds number increases heat transfer coefficient increases whereas friction factor decreases with Reynolds number and also can observe that increasing the volume concentration of CuO nano practical in the base fluid the heat transfer coefficient also increases.

   Fig 6.5 contours of surface heat transfer coefficient of plane tube

   Fig 6.6 Comparison of heat transfer coefficient of base fluid and Nanofluid from simulation analysis of laminar and turbulent flows.

   Fig 6.7 Comparison of heat transfer coefficient of base fluid and Nanofluid from simulation analysis of laminar flow

   Fig 6.7 Comparison of heat transfer coefficient of base fluid and Nanofluid from simulation analysis of turbulent flow

4. Effect of twist on friction factor and heat transfer enhancement:

Variations of heat transfer coefficient with Reynolds number and Friction factor with Reynolds number for the tube fitted with regularly spaced twisted inserts are shown in Figures for twist ratio H/D =5 and H/D=10. Fig 6.8 and 6.9 as shown that heat transfer for a given Reynolds number increases with decreasing twist ratio indicating enhanced heat transfer coefficient due to enhanced swirl flow as the Reynolds number increases for given twist ratio. The Nusselt number also increases with Reynolds number indicating enhanced heat transfer coefficient due to increased convection as flow increases.

The data obtained by simulation are matching with the literature value for plain tube with the discrepancy of less than 15% for Nusselt number for Sharma et al [2002] and

+8% for kishor [2005]) as shown in fig 6.10

Fig 6.8 contours of surface heat transfer coefficient of plane tube with twisted tape H/D=10

Fig 6.9 contours of surface heat transfer coefficient of plane tube with twisted tape H/D=5

Fig 6.10 Comparison of Nusselts number of base fluid for literature in laminar and turbulent flow

Fig 6.11 Comparison of Nusselts number of base fluid and Nanofluid for literature in laminar and turbulent flow

Fig 6.12 Comparison of heat transfer coefficient of base fluid and nano fluid for different twist ratios in laminar and turbulent flow

Fig 6.13 Comparison of heat transfer coefficient of base fluid and nano fluid for different twist ratios in laminar flow

Fig 6.14 Comparison of heat transfer enhancement of 0.1% CuO nano fluid for different twist ratios in laminar and turbulent flow

Fig 6.15 Comparison of friction factor of base fluid for different twist ratios in laminar and turbulent flow

Fig 6.16 Comparison of friction factor of base fluid and nano fluids for different twist ratios in laminar and turbulent flow

CONCLUSIONS

CFD simulation for the heat transfer augmentation in a circular tube fitted with twisted inserts in laminar flow conditions with the Reynolds from has been explained in this paper using fluent version 6.2.16.

The data obtained by simulation are matching with the literature value for plain tube with the discrepancy of less than 15%.for Nusselt number and 10 % for friction factor. The simulated results for the tube fitted with twisted inserts

are agreeing with the literature values 18% for Nusselt number and 15% for friction actor.

1. From the simulation analysis 0.1% volume concentration CuO Nanofluid is having heat transfer enhancement of 17.16% compared to base fluid at 2000 Reynolds number.

2. The heat transfer enhancement of CuO Nanofluid in plain tube with 0.1% volume concentration is 14.70% at 30000 Reynolds number and 19.12% at 50000 Reynolds number when compared to base fluid.

3. Further heat transfer enhancement of 0.1% Nanofluid with twisted tape insert of H/D = 5 is having 26% at 30000 Reynolds number and 33. 2% at 50000 Reynolds number compared to the same fluid flowing in plain tube.

4. The heat transfer enhancement of 0.1% Nanofluid with twisted tape insert of H/D = 5 is having 42.9% at Reynolds number 6000 and 86.01% at 50000 Reynolds number compared to base fluid flowing in a plain tube

REFERENCES

1. S.K.Saha A.Dutta Thermo hydraulic study of laminar swirl flow through a circular tube fitted with twisted tapes Trans. ASME Journal of heat transfer June 2001, Vol-123/ pages 417-427.

2. Zhi-Min Lin, Liang-Bi Wang Convective heat transfer enhancement in a circular tube using twisted tape Trans ASME journal of heat transfer Aug 2009,Vol-131/081901-1- 12.

3. Watcharin Noothong, Smith Eiamsa-ard and Pongjet Promvonge Effect of twisted tape inserts on heat transfer in tube 2nd joint international conference on sustainable Energy and Environment 2006 Bangkok, Thiland.

4. Paisarn Naphon Heat transfer and pressure drop in the horizontal double pipes with and without twisted tape in-sert 2005 Elsevier Ltd.

5. Smith Eiamsa-ard , Chinaruk Thianpong, Pongjet Promvonge Experimental investigation of heat transfer and flow friction in a Circular tube fitted with regularly spaced twisted tape elements International Communications in Heat and Mass Transfer Vol. 33, Dec 2006.

6. Ashis K. Mazumder, Sujoy K. Saha Enhancement of Thermo hydraulic Performance of Turbulent Flow in Rec- tangular and Square Ribbed Ducts With Twisted-Tape Inserts Journal of Heat Transfer AUGUST 2008, Vol. 130.

7. A Dewan1 , P Mahanta1, K Sumithra Raju1 and P Suresh Kumar Review of passive heat transfer augmentation