Numerical study of an exhaust heat recovery system using corrugated tube heat exchanger with twisted tape inserts

International Communications in Heat and Mass Transfer 57 (2014) 53–64

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Numerical study of an exhaust heat recovery system using corrugated tube heat exchanger with twisted tape inserts☆ Vamsi Mokkapati, Chuen-Sen Lin ⁎ Mechanical Engineering Department, University of Alaska Fairbanks, P.O. Box 755905, Fairbanks, AK, 99775-5905, USA

a r t i c l e

i n f o

Available online 15 July 2014 Keywords: Corrugated tube Twisted tapes Exhaust gas Heavy diesel generator Waste heat recovery Concentric tube heat exchanger

a b s t r a c t The purpose of this work is to investigate gas to liquid heat transfer performance of concentric tube heat exchanger with twisted tape inserted corrugated tube and to evaluate its impact on engine performance and economics through heat recovery from the exhaust of a heavy duty diesel generator (120 ekW rated load). This type of heat exchanger is expected to be inexpensive to install and effective in heat transfer and to have minimal effect on exhaust emissions of diesel engines. This type of heat exchanger has been investigated for liquid to liquid heat transfer at low Reynolds number by few investigators, but not for gas to liquid heat transfer. In this paper, a detail of heat transfer performance is investigated through simulations using computer software. The software is first justified by comparing the simulation results with the developed renowned correlations. Simulations are then conducted for concentric tube heat exchanger with different twisted tape configuration for optimal design. The results show that the enhancement in the rate of heat transfer in annularly corrugated tube heat exchanger with twisted tape is about 235.3% and 67.26% when compared with the plain tube and annularly corrugated tube heat exchangers without twisted tapes respectively. Based on optimal results, for a 120 ekW diesel generator, the application of corrugated tube with twisted tape concentric tube heat exchanger can save 2250 gal of fuel, $11,330 of fuel cost annually and expected payback of 1 month. In addition, saving in heating fuel also reduces in CO2 emission by 23 metric tons a year. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction In Alaska, there are about 180 villages using independent off grid diesel generators for many decades and are not equipped with exhaust heat recovery systems to take advantage of the energy in form of heat contained in the exhaust which is about 1/3 of fuel energy. Even a fraction of heat energy recovered may have a significant effect on heating fuel costs of cold region villages. From village power industry point of view, the major reasons for not installing exhaust heat recovery system is the effect on exhaust emissions and may cause maintenance difficulty resulting from soot accumulation and corrosion. In most of the Alaskan rural villages except in large power plants, maintenance technicians and engineers are not readily available when a maintenance problem rises (especially during winters), the shipping of equipment and traveling are also very expensive for isolated villages in the cold regions. One of the heat recovery systems which can match the needs is the vertically installed concentric tube heat exchanger with simple structure, which has been tested to capture the exhaust heat on a 120 kW diesel generator and no evidence has been found in emissions and maintenance. The heat exchanger is installed vertically in order to ☆ Communicated by W.J. Minkowycz ⁎ Corresponding author. E-mail address: [email protected] (C.-S. Lin).

http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.07.002 0735-1933/© 2014 Elsevier Ltd. All rights reserved.

reduce the soot accumulation. The water exiting the engine jacket is further heated in the current heat recovery system and supplied for space heating. The advantages of using jacket water for heat recovery are economical in installation, no additional pumping power required and no extra loop cost. The purpose of this work is to further improve the effectiveness of heat recovery by using heat augmentation techniques. A literature review has been conducted on concentric tube heat exchangers to determine the most suitable type of heat exchanger for cold region villages with higher effectiveness. Heat transfer augmentation techniques are classified into active and passive methods. Active methods require the direct external power, whereas passive methods do not require any direct external power. There are various techniques to reduce the thermal boundary layer thickness by improving good mixing of the fluids near walls and the center of the tube. Swirl flow generating devices provide chaotic mixing of the fluid and also good passive method of heat transfer augmentation. Many researchers conducted experiments and numerical simulations by inserting a wide range of twisted tapes, twisted coils and conical-rings into the inner tube of the concentric tube heat exchangers and proved that the thermodynamic efficiency of the system is increased. Heat transfer applications like waste heat recovery methods; refrigeration and air conditioning systems are using this type of swirl generators to enhance the heat recovery.

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Nomenclature CP d dt e f h H k m˙ Nu Pt p Pr Q q Re T t U v,u xi, xj

specific heat, J/kg K tube diameter, m twisted tape diameter, m enhancement Darcy friction factor heat transfer coefficient, W/m2 K total enthalpy, J turbulent kinetic energy, m2/s2, thermal conductivity, W/m K mass flow rate, kg/s Nusselt number twisted tape pitch, m pressure, kg/m s2 Prandtl number heat transfer rate, W heat flux, W/m2 Reynolds number temperature, K, constant time interval, s time, s quantity of parameters (e.g. velocity) fluid velocity, m/s cartesian co-ordinates, m

Greek symbols ∈ effectiveness ε dissipation, m2/s3 ρ density, kg/m3 μb viscosity, kg/m s eddy viscosity, kg/m s μt δ Kronecker delta ϑ viscous stress tensor τ Reynolds stress tensor

Subscripts c cold fluid h hot fluid l laminar t turbulent

Superscripts (ˉ) Reynolds average, average (˜) Favre average () Reynolds fluctuation () Favre fluctuation

Abbreviations ACT annularly corrugated tube CW corrugation width HE heat exchanger ID inner diameter OD outer diameter PT plain tube TT twisted tape

Some of the works using different types of swirl generators listed in the literature have been discussed. Al-Fahed et al. [1] conducted the experiments using oil as the working fluid for plain, micro fin and twisted-

tape insert tubes and concluded that the heat transfer is being increased with the increase in the twist ratio. Saha et al. [2] investigated the effect of regularly spaced twisted tape elements, experimentally in laminar and turbulent flow regime and reported that the heat transfer increases by 20–40%. Ray and Date [3] conducted numerical analysis inserting a twisted tape in the square duct and developed correlations for friction factor and Nusselt number and reported that there is a fair agreement between the simulation and the experimental results. Garcia et al. [4] conducted an experimental study on heat transfer in a plain tube with wire coil inserts in laminar-transition-turbulent regimes and stated that the heat transfer rate can be increased by up to 200% keeping pumping power constant. Eiamsa-ard et al. [5] conducted experiments and reported that Nusselt number is improved by 160% using full length twisted tape and 179% by inserting helical tape with and without rods [6] inside the tube when compared with the plain tube. Naphon and Sriromruln [7] conducted experiments by inserting the coiled wire in the plain tube and found that there is a significant effect of swirl in enhancing the rate of heat transfer. Chang et al. [8] investigated heat transfer in tube with broken twisted tape inserts experimentally and reported that the thermal performance is improved by up to 0.9–1.8 times of those tubes fitted with plain twisted tape. Heat transfer behavior with conical ring and twisted tape insertions was investigated experimentally by Promvonge and Eiamsa-ard [9] and reported that 367% enhancement in the heat transfer over the plain tube. Promvonge [10] in another report, mentioned that enhancement in the heat transfer is about 1.2–1.3 times when square wire coil is inserted in a circular tube. Promvonge [11] conducted experiments to investigate air flow friction and heat transfer characteristics in a round tube fitted with both coiled wire and twisted tape and obtained 200–350% enhancement in the heat transfer. Bharadwaj et al. [12] conducted experiments on spirally grooved tube with twisted tape and reported 140% of heat transfer enhancement over the plain tube. Rahimi et al. [13] also conducted numerical simulations with various twisted tapes and compared with the classic twisted tape and observed around 31% of enhancement in the heat transfer coefficient. Eiamsa-ard et al. [14] performed numerical simulations of swirling flow in circular tube by means of twisted tapes and reported that the mean heat transfer rates are about 73.6% higher than that of the plain tube. Instead of changing the type of swirl generating device, Thianpong et al. [15] changed the tube type and conducted experiments on a dimpled tube with classic twisted tape and found the enhancement up to 1.66 to 3.03 times that of the plain tube. Similarly, inserting the twisted tapes in the corrugated tube is expected to promote the generated swirl than that of plain tube and also can improve the rate of heat transfer. All the above studies along with the present study are summarized and presented in the Table 1. SolidWorks flow simulation software has been used to conduct the numerical study on the effect of swirl generated by twisted tapes of various configurations in the corrugated tube heat exchangers. This program is first verified with the renowned correlations for heat transfer coefficient in highly turbulent flow regime and a fair agreement of about ±10% between the simulation results and correlation prediction was observed. The simulation is then applied to diesel engine exhaust gas to liquid heat exchanger. Based on the literature search, this is the first study of this type of gas to liquid heat exchanger. 2. Diesel exhaust heat recovery system 2.1. Current system The idea has been tested by a power plant in Ruby (Alaska) with vertically installed concentric tube heat exchanger with annularly corrugated tube as inner tube for exhaust and outside tube is plain tube with jacket water which is exiting the diesel engine jacket to capture

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Table 1 Related studies from the literature. Paper

Tube

Tube diameter

Swirl generator

Type of HE

Increase

Al-Fahed et al. [1] Saha et al. [2] Ray and Date [3]b Garcia et al. [4] Eiamsa-ard et al. [5] Eiamsa-ard et al. [6] Naphon et al. [7] Chang et al. [8] Promvonge et al. [9] Promvonge [10] Promvonge [11] Bharadwaj et al. [12] Rahimi et al. [13]b Eiamsa-ard et al. [14]b Thianpong et al. [15] Present study

Microfin tube Plain tube Square duct Plain tube Plain tube Plain tube Microfin tube Plain tube Plain tube Plain tube Plain tube Grooved tube Plain tube Plain tube Dimpled tube Corrugated tube

14 mm 13 mm 9.5 mm × 9.5 mm 18 mm 19 mm 50 mm 8.92 mm 20 mm 48 mm 47.5 mm 47 mm 14.8 mm 17 mm (2.5–5.0) × P 22 mm 101.5 mm

Twisted tape Spaced twisted tapes Twisted tapes Wire coils Helical tape Spaced twisted tapes Coiled wire inserts Broken twisted tape Conical ring and twisted tape Coiled square wires Wire coil & twisted tape Twisted tape Modified twisted tapes Loose fit twisted tapes Twisted tape Twisted tape

Steam to oile Oild Waterf PG waterc Water to aire Waterc Waterc Aird Aird Aird Aird Waterd Waterc Waterf Waterc N2 to watere

225%a 20–40%a – 200% 160% 179% ~20% 0.9–1.8 times 367% 2.4 times 200–350% 400%a, 140% 2.49–1.96 times 73.6% 1.66–3.03times 235.3%

Note: the citations [1–15] reported are arranged in chronological order. a Laminar flow, all other flows are under turbulent region. b Numerical (CFD) study, all other reports are experimental investigations. c Liquid to Liquid HE. d Electric heat source to Liquid/Gas HE. e Gas to Liquid HE. f Constant heat flux boundary condition.

exhaust heat for space heating. Heat recovery system is attached to John Deere G4045-T300-HF458 Tier 3, 2010 model year, 1800 RPM, 120 ekW diesel generator (Fig. 1) [16]. The installation is proved to be retrofit and inexpensive. The annularly corrugated tube of the heat recovery unit installed in the Ruby power plant was manufactured by Penflex Corporation, and the following drawings and dimensions are provided on request and dimensional details are presented in Table 2. The outside tube (Shell) and inside tube (ACT) of the heat recovery system dimensions are presented in Fig. 2 and Table 2. The corrugated tube has 2.9 times the heat transfer surface area than that of plain tube and hence can improve the rate of heat transfer. Hence, this is one of the reasons behind replacing the corrugated tube in the place of plain tube.

2.2. Proposed system to improve heat transfer performance The goal of the present work is to maximize the heat recovery rate by optimizing the design of twisted tape. The controlled variables include the Reynolds number ranging from 40,000 to 77,000 of exhaust.

The physical size of the heat exchanger, twist ratio and exhaust back pressure change are the constraints.

3. Annularly corrugated tube heat exchanger with twisted tape inserts To improve the heat transfer rate of the heat recovery system, a modification of inserting a twisted tape into a corrugated tube is investigated. The modifications need to be retrofittable and be cost effective and also have negligible effect on emissions, maintenance and pumping power. Inserting twisted tape in the present heat recovery system has almost negligible effect on all the aspects mentioned earlier. The combined effect of the twisted tape and corrugations are expected to better improve the swirls of exhaust and also the rate of heat transfer. In order to select the optimal twisted tape insert, four different twisted tapes with different pitches are modeled and tested by inserting into the ACT HE. The pitch of the twisted tape is inversely proportional to the number of twists (Fig. 3). In general, it is believed that the more the number of twists are, the more swirl of the fluid can be achieved. Too many twists may increase

Fig. 1. Schematic diagram of the system.

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Table 2 Tube details. Tube

Pitch(m)

CW(m)

OD(m)

ID(m)

Thickness(m)

Plain Corrugated

– 0.0105

– 0.0075

0.102 0.122

0.1016 0.1016

0.61 × 10−3 0.61 × 10−3

the manufacturing difficulty as well as back pressure drop. The 3D model is presented in Fig. 4 for visualization. 4. Mathematical modeling Prediction of relationship between the heat transfer and swirling of the fluid inside the tubes is the main goal of the simulations. Simulations performed for annularly corrugated tube (ACT) heat exchanger with and without inserting various twisted tapes are mentioned in Table 3 and depicted in Fig. 5. Along with these simulations for plain tube heat exchanger for reference. In solving the numerical model, the following assumptions and boundary conditions are considered.

6. The inlet volume flow rate and temperature of water is 63 gpm (0.00397 m3/s) and 195 °F (90.55 °C) respectively. 7. The inlet volume flow rates and temperature of N2 are 0.3933 m3/s, 0.3343 m3/s, 0.2753 m3/s, 0.1966 m3/s and 510 °C (temperature of the engine exhaust). 8. The heat exchanger is installed vertically. 4.1. Validation The numerical simulations are performed to validate the heat transfer coefficient obtained from simulation with the existing correlations for Nusselt number. The agreement with the numerical analysis results and correlation (obtained from the experiment) results of Nusselt number values are within the 10% difference on the tube side. Fig. 6 presents the comparison between Numerical analysis, Dittus–Boelter's correlation, Gnielinski's correlation and Petukhov's correlation [17] in terms of dimensionless parameter Nusselt number (Nu), Reynolds number (Re) and Prandtl number (Pr). Nusselt number Nu ¼ hdh =k:

1. Steady operating conditions exist. 2. Radiation effects are negligible. 3. Nitrogen gas is used in the simulations in the place of exhaust gas for their similar thermal properties and treated as ideal gas. 4. The ambient pressure and the pressure at exit are both assumed to be at 1-atm. 5. Ambient temperature is assumed to be at 20 °C, with heat transfer coefficient of 15 W/m2K.

ð1Þ

Reynolds number Re ¼ ρvdh =μ:

ð2Þ

Prandtl number Pr ¼ C p μ=k:

Fig. 2. Exhaust heat recovery unit.

ð3Þ

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Fig. 3. Twisted tape dimensions.

Dittus–Boelter's correlation 0:8

Nu ¼ 0:024Re

Pr

0:3

:

ð4Þ

e ¼ 1 lim 1 U ρ T→∞ T

Gnielinski's correlation Nu ¼

ð f =8ÞðRe−1000Þ Pr
 1 þ 12:7ð f =8Þ0:5 Pr0:67 −1

−2

:

ð f =8Þ Re Pr
 C þ 12:7ð f =8Þ0:5 Pr0:67 −1

ð6Þ

C ¼ 1:07 þ 900=Re−½0:63=ð1 þ 10PrÞ: 4.2. Governing equations

• Classical time averaging for density and pressure, well known as Reynolds averaging Z

tþT t

e þ U ; T ¼ Constant Time interval: ρðt ÞU ðt ÞdtU ¼ U ″

ð8Þ Favre Averaged Navier Stokes (FANS) [19] equations, i.e., conservation equations can be written as follows. Continuity equation ð9Þ

Momentum equation

ð10Þ

where viscous stress tensor

SolidWorks CFD package is employed to perform the numerical analysis. The Favre Averaged Navier Stokes (FANS) equations are used in this CFD package to model the velocity, pressure fields, etc., and are combination of law of conservations of mass, momentum and energy for three dimensional models, also suitable for highly turbulent flow cases [18], which is density weighted time averaging Navier Stokes equation.

1 T→∞ T

t

  i ∂ðρuei Þ ∂ ρuei uej ∂p ∂ h þ ¼ ϑ þ τji þ ∂t ∂xi ∂x j ji ∂x j

where constant C in the above equation can be defined as

U ¼ lim

tþT

∂ρ ∂ðρuei Þ ¼ 0: þ ∂t ∂xi

Petukhov's correlation Nu ¼

Z

ð5Þ

where friction factor f in the above equation can be defined using Petukhov's correlation f ¼ ð0:79 ln ðReÞ−1:64Þ

• Density weighted time averaging for velocity and energy i.e., Favre averaging

0

U ðt Þ dtU ¼ U þ U :

ð7Þ

ϑij ¼ μ

! ∂uei ∂uej 2 ∂u − μ k δij þ 3 ∂xk ∂x j ∂xi

ð11Þ

and Reynolds stress tensor τij ¼ −ρu″i u″j :

ð12Þ

Boussinesq equation expressing the stress tensor τij ¼ μ t

! ek ∂uei ∂uej 2 ∂u 2 þ − δij − ρkδij : 3 ∂x j ∂xi 3 ∂xk

Fig. 4. 3D model of twisted tape swirl generator.

ð13Þ

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(A)

(B)

Fig. 5. Cross sectioned view of (A) ACT heat exchanger and (B) ACT heat exchanger with twisted tape.

Turbulent viscosity factor (fμ)

Total enthalpy " # i  ∂ ∂  ∂ 1 ∂p ∂ h  e ϑ þ τji e jH ¼ u ðρHÞ þ ρu −qlj −qtj þ ϑij u″i −ρu″j u″i u″i þ þ 2 ∂t ∂x j ∂x j ∂t ∂x j i ji 1 e þ 1u e H¼h e þ k; k ¼ ρu″i u″i : eu p ¼ ρRT; 2 i i 2

ð14Þ

 h  i2  20:5 : 1þ f μ ¼ 1− exp −0:025Ry pRffiffiffiffiT 2 ρk ρ k:y ;R ¼ RT ¼ : με y μ

ð17Þ

̃

where H = total enthalpy, h = Static enthalpy, k = turbulence kinetic energy, qlj = laminar heat flux vector and qtj = turbulent heat flux vector. The k-ε turbulence model is used to model the turbulent regime for the present study. For turbulent kinetic energy (k) ∂ρk ∂ ∂ ðρui kÞ ¼ þ ∂t ∂xi ∂xi

   μ ∂k R ∂ui þ τij μþ t −ρε þ μ t P B : σ k ∂xi ∂x j

The constants Cμ = 0.09, Cε1 = 1.44, Cε2 = 1.92, σε = 1.3, σk = 1 are defined empirically.

ð15Þ

For dissipation (ε) !   μ t ∂ε ε ρε 2 R ∂u f 1 τ ij i þ μ t C B P B −C ε2 f 2 þ C ε1 k σ ε ∂xi k ∂x j !3   0:05 2 f1 ¼ 1 þ ; f 2 ¼ 1−exp −RT : fμ

∂ρε ∂ ∂ ðρui ε Þ ¼ þ ∂t ∂xi ∂xi



μþ

ð16Þ

Table 3 Dimensional details of various twisted tapes. Twisted tape(TT)

Pitch(Pt)

Pt/dt

Number of twists

TT-1 TT-2 TT-3 TT-4

0.15 m 0.2 m 0.25 m 0.3 m

1.5 2 2.5 3

4 3 2.4 2

Fig. 6. Comparison of simulation results with existing correlation predictions.

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Table 4 Heat transfer comparison for PTHE with twisted tapes at a Reynolds number of 67,000.

Heat transfer(W) Enhancement (%)

PT_TT1

PT_TT2

PT_TT3

PT_TT4

8524.524 35.53

7989.685 27.03

7614.559 21.06

7516.491 19.5

4.3. Simulation procedure The basic flow equations are derived in finite volume method using integral approach. The conservative equations are discretized and solved throughout the model. The rate of increase in the quantity is equal to the sum of difference in the flux quantity and generation. ∂ ∂t

Z v

! ! UdV ¼ −∮s F :d S þ

Z Qdv v

ð18Þ

where U represents the quantity, V = Volume, S = Surface, F = Flux and Q = Source. The second order approximations of fluxes (F) are based on modified Leonard's Quadratic Upstream Interpolation for Convective Kinetics (QUICK) approximations and the Total Variation Diminishing (TVD) method. The pressure velocity coupling numerical algorithm i.e., Semi Implicit Method for Pressure Linked Equations (SIMPLE) is used to estimate the pressure field [17]. 4.4. Mesh independent study Three different meshes with total number of cells 387,705, 538,467 and 924,491 are initially considered and mesh independent study was performed to verify the results. The results of heat transfer rate are compared and the discrepancies between the results are less than 0.5%. The intermediate mesh level is chosen for all the simulations which saves about 41.76% of simulation time and gives similar results (0.5% deviation) when compared with the fine mesh.

Fig. 8. Twisted tape pitch versus the enhancement percentage.

5.1. Simulations of the PT HE with and without twisted tapes The simulation results show that the enhancement in the heat transfer of about 35.5%, 27.02%, 21.06% and 19.5% in the PT HE's with twisted tapes TT1, TT2, TT3 and TT4 respectively when compared with the PT HE without twisted tape. As expected the tape with maximum number of twists (TT1) provides more swirl and also yield to the highest rate of heat transfer (8.5 kW) among all the other tapes. The PT HE provides a heat transfer of 6.3 kW at a Reynolds number of 67,000. The rate of heat transfer for different twisted tape inserts in a PTHE is presented and compared with the PTHE in Table 4 and Fig. 7. It can be observed that the Nusselt number is increasing with the increase in the number of twists. The maximum Nusselt number at the maximum Reynolds number in the PTHE with TT1 is 200.

5. Numerical simulations and results To design the optimal twisted tape for exhaust heat recovery application, simulations have been conducted over a range of Reynolds numbers (Re) for the plain tube and corrugated tube with and without twisted tapes and compared to investigate the enhancement in the heat transfer coefficient (Nu) and rate of heat transfer (Q) as well. The Reynolds number range is from 33,000 to 68,000 for the gas in the inner tube.

Fig. 7. The Nu versus Re for the tubes with different twisted tape inserts.

5.2. Simulations of ACT heat exchangers with twisted tapes (TT1, TT2, TT3 and TT4) The simulations for ACT HE are conducted with similar boundary and inlet conditions which are used for the simulations with PT. The total heat recovered by ACT heat exchanger without twisted tape is around 11.48 kW. The further simulations with twisted tapes are compared with the heat transfer of the PT and ACT heat exchangers. The heat transfer behavior in annularly corrugated tube with different twisted tapes at four different Reynolds numbers of gas are investigated and numerical simulation results for rate of heat transfer are presented in Tables 5–8 and comparison between them is presented in Fig. 8. The heat transfer enhancement in ACT HE with TT1 is about 235% and 64.2% at a Reynolds number of 40,000 and about 205.43% and 67.25% at the Reynolds number of 77,700 when compared with the PT and ACT heat exchanger without TTs respectively. The enhancement percentage of heat transfer rate in ACT heat exchangers with TTs is observed to be higher at low Reynolds number. The flow trajectories of the gas inside PT and ACT HE without (Figs. 9, 10) and with TT1 (Figs. 11, 12) respectively are generated by the CFD program. The figures also represent the temperature variation of the gas from entrance to exit of HE with respect to the temperature scale beside the flow trajectories. Fig. 14 presents the direction of velocity vectors inside the ACT and ACT with TT1, the dots represent that there is no tangential velocity inside the tube. Fig. 13 presents the comparison between the heat transfer rate of ACT and PT HEs with various twisted tape pitches ranging from 0.15 m to 0.3 m at a nitrogen volume flow rate of 0.3933 m3/s.

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Fig. 9. Flow trajectories with temperature distribution of the gas inside the plain tube.

Fig. 10. Flow trajectories with temperature distribution of the gas inside the ACT.

Fig. 15 presents the variation in the Nusselt number in the heat exchangers at different Reynolds numbers for ACT with different twisted tape inserts. The ACT with twisted tape (TT1) has the maximum Nusselt number of 201.75 at Reynolds number 77,700 when compared with the other ACT heat exchangers with and without twisted tape

inserts. TT1 has more number of twists and can generate more swirls in the ACT than that of other TTs and hence the heat transfer coefficient is comparatively high. Figs. 16 and 17 present the contour plots of tangential velocity inside the plain and ACT without and with different twisted tapes

Fig. 11. Flow trajectories and temperature distribution of the gas inside the PT with TT-1.

Fig. 12. Flow trajectories and temperature distribution of the gas inside ACT with TT-1.

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Fig. 13. Heat transfer rate versus pitch of the twisted tape inserts in the ACTHE and PTHE.

respectively. High tangential velocity is observed in the ACT with the similar twisted tape and fluid flow conditions, which in other words, also confirms more swirling. 5.3. Effectiveness (∈) of PT and ACT heat exchangers Effectiveness is a measure of heat transfer performance of a heat exchanger and is denoted by ∈. Effectiveness of a heat exchanger can be defined as the ratio of the total amount of heat that has been exchanged to the maximum possible heat transfer rate [20].



where ¼ C c T c;o −T c;i



Q ∈¼  Q max    ð19Þ ¼ C h T h;i −T h;o ; Q max ¼ C min T h;i −T c;i

C c ¼m˙c Cpc ; C h ¼m˙h Cph ; C min ¼ Minimum of C c and C h

Using the above equations the Effectiveness of the PT and ACT (with and without twisted tapes) heat exchangers under similar conditions are calculated from the simulation results. The effectiveness of ACT HE with and without TT1 is about 23.67% and 14.35%, whereas the effectiveness of PT HE with and without TT1 is about 10.67% and 8.17% respectively.

61

(6.3 kW) of plain tube (PTHE). This may have resulted from the more surface area of the ACTHE. The results also show that the twisted tape (TT) inserts can improve heat transfer related performance parameters, such as Nusselt number (Fig. 7), heat transfer enhancement (Fig. 8), and heat transfer rates (Tables 4 to 8) against both plain tube and corrugated tube. This can attribute to the swirls caused by the twist tape inserts, which can be found in Figs. 9 to 12. Figs. 9 and 11 show that the flow swirl (represented by stream lines) of plain tube with TT-1 insert is much higher than that of a plain tube without a TT. A similar observation is found in Figs. 10 and 12 for corrugated tube. From Tables 4 and 6, it can be found that, for about the same level of Reynolds number, the enhancement percentages resulted from TT insertion for corrugated tube (Table 5. Enhancement from 23.3% to 67.26%) is significantly higher than that for plain tube (Table 4. Enhancement from 19.5% to 35.53%). The cause may be that the corrugated tube help in promoting the swirl and as a result heat transfer rate is largely increased. According to Figs. 8 and 9, the flow trajectory inside the plain tube and annularly corrugated tube is very similar to each other; the advantages of corrugations are not completely utilized. Inserting the twisted tape in the plain tube and corrugated tubes, the swirling of the fluid can be achieved and a significantly more amount of the temperature is being dropped in the corrugated tube (with TT1) than that in the plain tube with TT1, this can be observed in Figs. 11 and 12. From observation, the heat transfer rate increases as the pitch of the TT decreases (Fig. 13). This may be a result that the shorter the pitch means the more the twists for the same length HE. The more twists may guide the fluid to travel more turns along the HE and cause higher tangential velocity inside the HE (Figs. 16 and 17). From Fig. 8, the enhancement percentage is about 235.32% and 205.43% at a Reynolds number of 40,000 and 77,000 respectively i.e. the twisted tapes are performing more effectively at lower flow rates in annularly corrugated tube. From Table 9, the effectiveness of PTHE is about 8.17% and when the PT is replaced by ACT the effectiveness is increased to 14.34%. But inserting twisted tape in the ACT HE, the effectiveness is drastically increased to 23.6% which is about 9.26% more than that of the PTHE. According to simulation results, the maximum (~ 67.25%) rate of heat transfer was observed when TT-1 is inserted in the current trial system of Ruby power plant, Alaska.

6. Discussion of results

7. Economic analysis

Simulation results show that, for the same heat exchanger dimension (i.e., height and diameter), the heat transfer rate (11.4 kW) of an annularly corrugated heat exchanger (ACTHE) is higher than that

For the Ruby village power plant diesel generator equipped with ACT vertical HE is able to recover 11.48 kW of heat from the diesel engine exhaust. The present ACT HE unit cost is $2,215; approximated cost for the

Fig. 14. Velocity vectors inside ACT and ACT with TT1 at tube exit.

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Similarly, economic analysis results for ACT with TT1, which show the most improvement among all the TTs are also listed in row 2 of Table 10. Saving fuel not only saves money but also reduces the amount of CO2 released into the atmosphere. About 10.15 kg of CO2 is released by burning 1 gal of heating oil [21]. Using ACT HE with twisted tape can save up to 5,636.5 gal per year which is 2,266 gal more than that of ACT HE (presently using HE), while also reducing emissions of CO2 by up to 23 metric tons a year. 8. Conclusions

Fig. 15. Nusselt number versus Reynolds number for ACT.

twisted tape (TT1) is $500 (estimated by a local machine shop). Simulation results show that using twisted tape with a pitch of 0.15 m can enhance the heat recovery rate by up to 67.25%. The cost of the twisted tape = $500. The total system cost = $2,715 (i.e., unit cost + tape cost = $2,215 + $500). Heating value for the fuel at 75% boiler efficiency = 101,250 BTU/ gal. Total heat recovery rate of ACT HE = 11.48 kW = 39,171.36 BTU/h. Heat recovered per year = 39,171.36 BTU/h × 24 h/day × 363days/ year = 341.22 MBTU/year. Based on $5.00 per gallon of fuel cost, which is common for rural Alaskan villages, the estimated payback period for ACT heat exchanger with TT1 is about 1 month and practically has no effect in the maintenance. A summary of the economic analysis is listed in the first row of Table 10.

A study to improve the effectiveness of a diesel exhaust heat recovery system (concentric tube heat exchanger) of Ruby, Alaska has been conducted. The improved system is the original heat exchanger inserted with a twisted tape. The goal of the present work is to maximize the heat recovery rate by optimizing the design of twisted tape insert. The physical size of the heat exchanger, twist ratio and exhaust back pressure change are the constraints. In this study, the outer tube is a plain tube and the inner tube is an annularly corrugated tube with and without twisted tape inserts. The following paragraphs summarize the findings obtained from this study. 1. The effect of twisted tape insert was found to cause significant improvement in heat transfer performance of the plain tube heat exchanger. Among all the twisted tapes tested (TT1, TT2, TT3 & TT4) the one with lower pitch i.e. TT1 (~0.15 m) has more effect on the heat transfer enhancement which is about 35.5%. 2. The corrugated tube heat exchanger improves the rate of heat transfer up to 82% and 35% when compared with the plain tube without and with TT1 respectively. 3. The corrugated tube heat exchanger with twisted tape insert has more heat transfer rate when compared with the corrugated tube alone. The effect of twisted tape insert was found to cause significant improvement in the heat transfer performance. The heat transfer rate increased by inserting the twisted tapes (TT1, TT2, TT3 and TT4) in ACT heat exchanger is about 67.25%, 49.89%, 33.83% and 23.3% respectively when compared with the ACT HE without twisted tape.

Fig. 16. Tangential Velocity Contour plot at exit of PTHE without and with TT (1–4).

Fig. 17. Tangential Velocity Contour plot at exit of ACTHE without and with twisted tapes (1–4).

V. Mokkapati, C.-S. Lin / International Communications in Heat and Mass Transfer 57 (2014) 53–64 Table 5 Simulation results of ACT heat exchanger with twisted tapes at Re = 77,700.

Heat transfer(W) e (%)-PT⁎ e (%)-ACT⁎⁎

Table 9 Heat exchanger effectiveness.

ACT_TT1

ACT_TT2

ACT_TT3

ACT_TT4

HE

Without TT

TT1

TT2

TT3

TT4

19211.17 205.52 67.26

17217.42 173.73 49.9

15372.72 144.4 33.83

14162.57 125.17 23.3

PT ACT

8.17% 14.34%

10.68% 23.6%

10% 21.29%

9.52% 19.05%

9.4% 17.57%

⁎ The e(%) -PT refers the enhancement percentage when compared with PT HE without TTs. ⁎⁎ The e(%) -ACT refers the enhancement percentage when compared with ACT HE without TTs.

Table 6 Simulation results of ACT heat exchanger with twisted tapes at Re = 66,200.

Heat transfer(W) e (%)-PT⁎ e (%)-ACT⁎⁎

ACT_TT1

ACT_TT2

ACT_TT3

ACT_TT4

16828.29 196.63 63.41

15391.3 171.3 50.98

13952.96 145.95 38.59

12887.44 127.17 27.13

⁎ The e(%) -PT refers the enhancement percentage when compared with PT HE without TTs. ⁎⁎ The e(%) -ACT refers the enhancement percentage when compared with ACT HE without TTs.

4. The heat transfer rate in corrugated tube heat exchanger with twisted tape insert is 67% more than that of corrugated tube alone. When the same twisted tape is inserted in the plain tube heat exchanger, the heat transfer rate is about 35.3% more than that of tube alone. This clearly states that corrugated tube is promoting the effect of swirl than that of plain tube. 5. Using ACT HE with twisted tape can save up to 5,636.5 gal of fuel per year which is 2,266 gal more than that of presently installed heat recovery system and also reducing emissions of CO2 by up to 23 metric tons annually. 6. Payback period for the proposed exhaust heat recovery system is about 1 month in comparison with the currently installed setup which is 1.5 months. 7. The pressure drop of the exhaust gas in this heat recovery system is about 0.257 psi (1.77 kpa), which conveys that the effect of inserting twisted tape on the engine performance is minimal. Based on this study, the proposed exhaust heat recovery system with TT1 is simple, relatively inexpensive, easy to maintain and no practical effect on exhaust emissions. Also it has a good economic effect on Ruby diesel power application. If the system is applied to all the diesel

Table 7 Simulation results of ACT heat exchanger with twisted tapes at Re = 53,000.

Heat transfer(W) e (%)-PT⁎ e (%)-ACT⁎⁎

ACT_TT1

ACT_TT2

ACT_TT3

ACT_TT4

14546.09 203.44 60.89

13650.8 184.77 50.98

12530.05 161.38 38.59

11494.28 139.78 27.13

⁎ The e(%) -PT refers the enhancement percentage when compared with PT HE without TTs. ⁎⁎ The e(%) -ACT refers the enhancement percentage when compared with ACT HE without TTs.

Table 8 Simulation results of ACT heat exchanger with twisted tapes at Re = 40,000.

Heat transfer(W) e (%)-PT⁎ e (%)-ACT⁎⁎

63

ACT_TT1

ACT_TT2

ACT_TT3

ACT_TT4

11765.7 235.32 64.294

10821.17 208.40 51.105

10379.16 195.80 44.93

9401.002 167.93 31.27

⁎ The e(%)-PT refers the enhancement percentage when compared with PT HE without TTs. ⁎⁎ The e(%)-ACT refers the enhancement percentage when compared with ACT HE without TTs.

Table 10 Economic analysis. Heat exchanger

ACT (current installation)

ACT with TT1(proposed installation)

Initial unit cost Heat recovered annually Annual fuel savings Annual savings Estimated payback period

$2,150 341.22 MBtu/year

$2,650 570.69 MBtu/year

3,370.1 gal/year $16,850/year 1.5 months

5,636.5 gal/year $28,181/year 1 months

generators in rural Alaskan villages, the potential effect on economy of diesel power industry may reach to a savings of $4 million per year based on 370,000 MW-h power consumption in 2007 [22]. Acknowledgments The authors would like to gratefully acknowledge the support of Alaska Center for Energy and Power, Mechanical Engineering Department, College of Engineering and Mines at University of Alaska Fairbanks. References [1] S. Al-Fahed, L.M. Chamra, W. Chakroun, Pressure drop and heat transfer comparison for both microfin tube and twisted-tape inserts in laminar flow, Exp. Thermal Fluid Sci. 18 (1999) 323–333. [2] S.K. Saha, A. Dutta, S.K. Dhal, Friction and heat transfer characteristics of laminar swirl flow through a circular tube fitted with regularly spaced twisted-tape elements, Int. J. Heat Mass Transf. 44 (22) (2001) 4211–4223. [3] S. Ray, A.W. Date, Friction and heat transfer characteristics of flow through square duct with twisted tape insert, Int. J. Heat Mass Transf. 46 (5) (2003) 889–902. [4] A. García, P.G. Vicente, A. Viedma, Experimental study of heat transfer enhancement with wire coil inserts in laminar-transition-turbulent regimes at different Prandtl numbers, Int. J. Heat Mass Transf. 48 (21–22) (2005) 4640–4651. [5] S. Eiamsa-ard, P. Promvonge, Enhancement of heat transfer in a tube with regularlyspaced helical tape swirl generators, Sol. Energy 78 (4) (2005) 483–494. [6] S. Eiamsa-ard, C. Thianpong, P. Promvonge, Experimental investigation of heat transfer and flow friction in a circular tube fitted with regularly spaced twisted tape elements, Int. Commun. Heat Mass Transfer 33 (10) (2006) 1225–1233. [7] P. Naphon, P. Sriromruln, Single-phase heat transfer and pressure drop in the microfin tubes with coiled wire insert, Int. Commun. Heat Mass Transfer 33 (2) (2006) 176–183. [8] S.W. Chang, T.L. Yang, J.S. Liou, Heat transfer and pressure drop in tube with broken twisted tape insert, Exp. Thermal Fluid Sci. 32 (2) (2007) 489–501. [9] P. Promvonge, S. Eiamsa-ard, Heat transfer behaviors in a tube with combined conical-ring and twisted-tape insert, Int. Commun. Heat Mass Transfer 34 (7) (2007) 849–859. [10] P. Promvonge, Thermal performance in circular tube fitted with coiled square wires, Energy Convers. Manag. 49 (5) (2008) 980–987. [11] P. Promvonge, Thermal augmentation in circular tube with twisted tape and wire coil turbulators, Energy Convers. Manag. 49 (11) (2008) 2949–2955. [12] P. Bharadwaj, A.D. Khondge, A.W. Date, Heat transfer and pressure drop in a spirally grooved tube with twisted tape insert, Int. J. Heat Mass Transf. 52 (7–8) (2009) 1938–1944. [13] M. Rahimi, S.R. Shabanian, A.A. Alsairafi, Experimental and CFD studies on heat transfer and friction factor characteristics of a tube equipped with modified twisted tape inserts, Chem. Eng. Process. Process Intensif. 48 (3) (2009) 762–770. [14] S. Eiamsa-ard, K. Wongcharee, S. Sripattanapipat, 3-D numerical simulation of swirling flow and convective heat transfer in a circular tube induced by means of loose-fit twisted tapes, Int. Commun. Heat Mass Transfer 36 (9) (2009) 947–955. [15] C. Thianpong, et al., Compound heat transfer enhancement of a dimpled tube with a twisted tape swirl generator, Int. Commun. Heat Mass Transfer 36 (7) (2009) 698–704. [16] Ruby Power Plant Exhaust Heat Recovery Testing, 2011.

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[17] Introduction to Heat Transfer, by Frank P. Incropera, David P. DeWitt, Third edition John Wiley & Sons, Inc., 1996 [18] SolidWorks Flow Simulation 2012, Technical Reference, 2012. [19] Turbulent Flows: Fundamentals, Experiments and Modeling by G. Biswas, V. Eswaran, Science, CRC Press, 2002. (456 Pages). [20] Fundamentals of Heat Exchanger Design by Ramesh K. Shah and Dusan P. Sekulic, John Wiley & Sons, Inc., 2003

[21] U.S. Energy Information Administration, www.eia.gov/oiaf/1605/coefficients.html 2011. [22] Capture of heat energy from diesel engine exhaust by Lin CS, Final Report Prepared for National Energy Technology Laboratory, DOE Award # DE-FC26-01NT41248, November 2008.