CFD Analysis of a Cross-flow Heat Exchanger with Different fin thickness

Transcription

1 International Journal o Dynamics o Fluids. ISSN Volume 13, Number 2 (2017), pp Research India Publications CFD Analysis o a Cross-low Heat Exchanger with Dierent in thickness K.Ravikumar 1, Ch.Naga Raju 2, Meera Saheb 3 1 Assistant Proessor, V.R.Siddhartha Engineering College,. 2 Proessor, V.R.Siddhartha Engineering College. 3. Proessor, JNTU Kakinada, Abstract Eiciency o heat exchanger and its dimensions are ones o the most important parameters to consider in engineering design. The size o heat exchanger can be more compact by introducing the ins to increase the heat transer rate between the heat exchanger surace and the surroundings. Dierent engineering methods are used in heat exchanger design process. The proper correlations or modeling and simulation tools are oten applied to receive the general recommendation at early stages o exchanger study. The perormance o the in-tube heat exchanger or dierent in thickness is calculated. To give indications about the accuracy o numerical outcome, the most popular correlations are evaluated and results obtained rom Ansys CFX program are veriied. Analyzing the output, it seems that the implementation o the CFD model oers particular beneits especially when minor modiication are applied to the in surace or which the correlation equations are not deined. The objective o the present work is to simulate the 3D geometry or cross low smooth and inned tube heat exchanger with using hot water inside the tube and cooling air outside the tube by using computational luid dynamic (ANSYS-FLUENT 15). The enhancement o heat transer has been introduced in many ields o industrial and scientiic applications. For the simulation, purpose a symmetric view o the simpliied geometry o the heat exchanger is made using solid works sotware. Keywords: Heat exchanger, Fin thickness, CFX, CFD, 1. Introduction

2 346.Ravikumar, Ch.Naga Raju, Meera Saheb Heat exchangers are devices used to transer heat energy rom one luid to another. Typical heat exchangers experienced by us in our daily lives include condensers and evaporators used in air conditioning units and rerigerators. Boilers and condensers in thermal power plants are examples o large industrial heat exchangers. There are heat exchangers in our automobiles in the orm o radiators and oil coolers. Heat exchangers are also abundant in chemical and process industries. Dierent heat exchangers are named according to their applications. For example, heat exchangers being used to condense are known as condensers; similarly heat exchangers or boiling purposes are called boilers. Perormance and eiciency o heat exchangers are measured through the amount o heat transerred using least area o heat transer and pressure drop. A better presentation o its eiciency is done by calculating over all heat transer coeicient. Pressure drop and area required or a certain amount o heat transer, provides an insight about the capital cost and power requirements (Running cost) o a heat exchanger. Usually, there is lots o literature and theories to design a heat exchanger according to the requirements. A good design is reerred to a heat exchanger with least possible area and pressure drop to ulill the heat transer requirements. Cross low heat exchangers may be inned or corrugated and may be used in single-pass or multipass modes o operation. Flow passages associated with compact heat exchangers are typically small, and the low is usually laminar. 2. Fin-tube cross-low heat exchanger geometry The analysis o heat transer rom inned suraces involves solving second-order dierential equations and is oten a subject o researches including also the variable heat transer coeicient as a unction o temperature or the in geometrical dimensions. In general, the study o the extended surace heat transer compromises the movement o the heat within the in by conduction and the process o the heat exchange between the in and the surroundings by convection [18]. For the ideal case, the optimized proile o the symmetrical radial in o least material can be ound rom the generalized dierential equation [19]. It leads to the parabolic in shape or which the heat lux is less sensitive to the variation o the tip temperature than in the case o rectangular and trapezoidal in proiles. In practice, low mal distribution is common during the air low and inluences the perormance o heat exchangers. The analysis and design o heat exchangers consider problems in which the temperature o the luid changes as it lows through a passage as a result o heat transer between the wall and the luid. For heat transer analyses, at least the ollowing heat transer surace geometrical properties are needed on each side o a two-luid exchanger: minimum ree-low area, core rontal area, heat transer surace area which includes both primary and in area, hydraulic diameter, and low length.

3 CFD Analysis o a Cross-low Heat Exchanger with Dierent in thickness 347 Figure 1: Fin-tube geometry, with minimum cross-sectional area Surace area o one sector consists o in and tube are deined as 1 A D D D Surace area o ins: Surace area o tube between ins: Total surace area: A T DS At DS Reynolds number, maximum luid velocity and Nusselt number is deined as max D Re D max m A o hd Nu k The heat exchanger characteristic dimensions are written or dierent in thickness is tabulated in Table 1: (1) (2) (3)

4 348.Ravikumar, Ch.Naga Raju, Meera Saheb Fin version R D /2 Table:1: Heat exchanger characteristic dimensions mm R=D/2 mm t mm P mm Pt mm a b c mm 3.Correlation or external heat transer in in-tube cross low heat exchanger The value o heat transer depends on local luid velocity, luid properties and details o the tube bank geomentry. Correlations that allow calculating average heat transer coeicient eatures. ' h are derived rom experimental data and take into account geometrical 3.1. Recommended correlation to calculate the average Nusselt number or staggered tube banks by Engineering Sciences Data Unit [21] The correlation can be applied or Reynolds number range s X t l, X l : Re 410 and s X t 3 Nu Re.Pr. F1. F2 l Xl (4) Where X t Xl Pt -transverse tube pitch longitudinaltubepitch, F1-Factor or luid property variation F2-Factor or number o in-tube rows (F2=0.71 is applied or all correlations) 0.98 For our or more rows 0.93 For three rows 0.81 For two rows 0.73 For one row

5 CFD Analysis o a Cross-low Heat Exchanger with Dierent in thickness Correlation o Briggs and Young [19], [22], [23] s s Nu Re Pr. l (5) The correlation is based on experimental data or eight row tube banks laid out on equilateral triangular pitch and 1.10 Re 1.810,11.02<D<40.32mm,1.38<l<16.43mm,0.33mm<δ<1.96mm,0.76mm<s<2.72mm,24.2 1mm<Xt<109.02mm, Eective heat transer coeicient Eective heat transer coeicient, or the air lowing outside and at right angles to the axis o a bank o inned pipes,can be represented approximately by the dimensional equation[24]: A ' At h h A T (6) 0.6 IN X t h D X t D (7) 4. Mean temperature Coeicient and heat transer in heat exchanger Total heat transer can be calculated taking into consideration in eiciency: ' t Q ht A A h TA Where (8) -in eiciency ' h -eective heat transer coeicient To evaluate the heat transer, it is necessary to ind the eective mean temperature dierence, T. since the luid temperature changes in luid low through the tube bank, the luid temperature dierence TFluid can be calculated rom energy exchanged as: Q ht A A m c T Where t Fluid (9)

6 350.Ravikumar, Ch.Naga Raju, Meera Saheb T And or T IN T T T T T 0 OUT 0 OUT T0 T ln T T 0 OUT IN IN (10) TFluid TIN TOUT Ater transormation h A At TFluid T mc h A At TOUT TIN T mc T T T IN 0 h A At 1exp mc h A At mc Having calculated eective mean temperature dierence, T, average heat transer coeicient, h, and in eiciency, η, the rate o heat transer can be ound rom Eq.(11) The in eiciency value η can be achieved rom Equation [20] (11) (12) (13) (14) 2h tanh. k 2h. k (15) Where D D D ln 2 D D (16)

7 CFD Analysis o a Cross-low Heat Exchanger with Dierent in thickness Results o heat transer calculations Calculations are done or circular in-tube heat exchanger. Three-dimensional models are perormed to ind heat transer characteristics between a inned tube and the air or dierent in shapes in order to ind the heat transer rate between the air and the in material during the air low in the cross low heat exchanger. The model allows considering the heat transer in three directions. The output is compared with the results received rom the correlation ormula. Using the described correlation, the heat transer is determined based on deined the mass low rate (inlet velocity 4.0 m/s), inlet temperature o the luid (300 o C) and the internal tube surace temperature (70 o C). Values o Vmax, Nu or one row, eective heat transer coeicient and luid outlet temperature, received rom correlation unctions or each in version, are written in Table 2. Table2. Heat exchanger surace and low parameters or in version (a), (b) and (c). Correlation Eq.(4) Correlation Eq.(5) Correlation Eq.(7) (a) (b) (c) (a) (b) (c) (a) (b) (c) V_max Nu (one row) h w/(m 2 K) TOUT, ºC Numerical analysis is also carried out to examine modiied inned tube heat exchangers and the inluence o the in thickness on the heat transer. The numerical outcome o heat transer coeicient orm 3D model is compared to the results received rom the correlations or the in-tube heat exchanger o uniorm in thickness. Correlations are used to check the numerical calculation o the heat transer and its accuracy in relation to in shape modiications. Results are presented in Table 3, where T _ correlat T _correlat.100% T _ correlat (17)

8 352.Ravikumar, Ch.Naga Raju, Meera Saheb Table 3. Comparison between numerical calculations and correlations or in version (a),(b) and (c) Correlation Eq. (4) Correlation Eq. (5) Correlation Eq. (7) (a) (b) (c) (a) (b) (c) (a) (b) (c) T_Correlatº C T_modelºC % 1.66 % 3.63 % % % % % % % 4.1 Mesh Generation Mesh generation is very important step o pre-processing stage because it its the limits o computational domain. Many engineering applications need mesh generation that is appropriate or the solving o 3D Navier-Stokes equations. In the present work, tetrahedron element is used or 3D geometry mesh. Good mesh is recognized rom its generated cells number. For a complex geometry, increase the cells number will increase the resolution and the accuracy, but also this increase will be opposed by increase in computer memory, need or high processor and take more time to complete the solution. At last there must be an optimization between the number o cells generated and the time consumed or the solution process. For the present work, the mesh generation is shown in Figure 2. Figure 2: Mesh generation o the present work geometry

9 CFD Analysis o a Cross-low Heat Exchanger with Dierent in thickness GOVERNING EQUATIONS The undamental basis o most o CFD problems are the solutions o (mass, momentum and energy) equations, as well as the transport equation or turbulent viscosity and its scale. These are in steady state and have been stated below in simple orm. For turbulent low[4]: 5. THE BOUNDARY CONDITIONS A. Inlet Boundary Conditions The velocity o the inlet air is limited with a values o (1, 2, 3, and 4) m/s, while the volume low rate o tube side liquid is limited with a values o (2, 3, 4, 5 and 6) L/min and the temperature o inlet air is the room temperature, while the temperature o tube side liquid is limited with a values o (50, 60, 70 and 80) C. B. Outlet Boundary Conditions The outlet or air side and tube side luid is speciied as pressure outlet and it s represented by the atmospheric pressure. 6. RESULTS AND DISCUSIONS The numerical simulation is done by ANSYS FLUENT 15. sotware to show both the low ield and heat transer o the present models. Many cases are studied. Three cases are discussed in the ollowing sections. Same boundary conditions are used in the three cases, which are (air velocity o 1 m/s, water inlet temperature and low rate o (80 C) and (2 L/min) respectively).

10 354.Ravikumar, Ch.Naga Raju, Meera Saheb A. Temperature Contours Figure 3 shows a 3D simulation o temperature distribution in the test section, igures 4. and 5. reveal temperature contours o smooth tube with water and integral inned tube with water. From these igures, it is noted that there is a gradient o temperature distribution along with test tube and the temperature dierence are clearly appear in all cases. Also, it is clear rom the igures that the temperature gradient o inned tube is higher than that o smooth tube. This means that ining have a substantial eect on increasing the temperature dierence inside the test tube. Figure 3(a): shows contours o Total Temperature (mixture) (K) B. Velocity and Vectors Contours Figure 4. (a) shows a longitudinal section o velocity contour, rom igure, the velocity o water inside the tube is constant due to stability o water low rate. Figure 4. (b) demonstrate a cross section o velocity contour, this igure represent the behavior o air through the test section, beore the test tube the velocity o air are constant, the air velocity are increased during it across through the passes o test tube, ddies are ormed behind the test tube and turbulence is increased.

11 CFD Analysis o a Cross-low Heat Exchanger with Dierent in thickness 355 Figure 4 (a) longitudinal section o velocity contour Figure 4 (b) cross section o velocity contour

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16 360.Ravikumar, Ch.Naga Raju, Meera Saheb 7. CONCLUSIONS The main objective o this research is to determine numerically the perormance o the heat transer process in a single row in-tube cross low heat exchanger or dierent in conigurations. The most popular correlations are applied or heat transer evaluation. For Briggs and Young correlation, the heat transer decreases with the in thickness increase. The opposite results are seen or the other correlations. The heat transer is also analyzed by means o numerical computation. The results are veriied with the known correlations or circular ins o constant thickness. Analyzing

17 CFD Analysis o a Cross-low Heat Exchanger with Dierent in thickness 361 the output received rom numerical calculations with those gathered rom correlations, it seems that the dierences are within the standard deviation and numerical techniques can predict heat transer coeicients with acceptable accuracy. The use o the CFD model oers particular beneits especially when minor modiication are applied to the in surace or which the correlation equations are not deined, or instance in thickness modiication. However, comparative analyses are still required and the numerical model should be examined, veriied with proper correlations or experimental values. Reerences [1] P. Wais, One row in heat exchanger numerical optimization, Proceedings o International Congress on Thermodynamics, 4-7 Sept Poznan, (2011) [2] R. K. Shah, D. P. Sekulic, Fundamentals o Heat Exchanger Design, Wiley, [3] P. Wais, Extended Suraces (Fins and Pins), in: R.B. Hetnarski, Encyclopedia o Thermal Stresses, Vol 3, Springer, Dordrecht, 2014, [4] J. Y. Jang, M. C Wu., W. J. Chan, Numerical and experimental studies o three dimensional plate-in and tube heat exchangers. International Journal o Heat and Mass Transer 14 (1996) [5] P. Wais, J. Taler, Fin shape optimization in tube heat exchangers by means o CFD program, 2nd International Conerence on Engineering Optimization, Sept. 6-9, Lisbon, Portugal, (2010) [6] P. Wais, Fluid low consideration in in-tube heat exchanger optimization, Archives o Thermodynamics, 31, (2010) [7] W. M. Yan, P. J. Sheen, Heat transer and riction characteristics o in-andtube heat exchangers, International Journal o Heat and Mass Transer 43, (2000) [8] M. S. Mon, U. Gross, Numerical study o in-spacing eects in annular-inned tube heat exchangers. International Journal o Heat and Mass Transer 47, (2004) [9] R. Romero-Mendez, M. Sen, K. T. Yang, R. McClain, Eect o in spacing on convection in a plate in and tube heat exchanger, International Journal o Heat and Mass Transer 43, (2000) [10] P. Wais, Fin-tube heat exchanger perormance or dierent louver angles, Zeszyty Naukowe Politechniki Rzeszowskiej Mechanika 86, (2014) [11] P. Wais, Inluence o in thickness and winglet orientation on mass and thermal eiciency o cross-low heat exchanger, Applied Thermal Engineering 102, (2016)

18 362.Ravikumar, Ch.Naga Raju, Meera Saheb [12] A. Erek, B. Ozerdem, L. Bilir, Z. Ilken, Eect o geometrical parameters on heat transer and pressure drop characteristics o plate in and tube heat exchangers. Applied Thermal Engineering 25, (2005) [13] S. Y. Yoo, H. K. Kwonb, J. H. Kima, A study on heat transer characteristics or staggered tube banks in cross-low. Journal o Mechanical Science and Technology 21, (2007) [14] D. Taler, Methods or obtaining heat transer correlations or plate inned heat exchangers using experimental and CFD simulated data, Archives o Thermodynamics 25, (2004) [15] D. Taler, P. Ocłoń, Determination o heat transer ormulas or gas low in inand-tube heat exchanger with oval tubes using CFD simulations, Chemical Engineering and Processing 83, (2014) [16] D. Taler, P. Ocłoń, Thermal contact resistance in plate in-and-tube heat exchangers, determined by experimental data and CFD simulations, International Journal o Thermal Sciences 84, (2014) [17] P. Ocłoń, S. Łopata, M. Nowak, A. C. Benim, Numerical study on the eect o inner tube ouling on the thermal perormance o high temperature inand-tube heat exchanger, Progress in Computational Fluid Dynamics, An International Journal, 5, (2015) [18] P. Wais, Fin-tube heat exchanger optimization, in: J. Mitrovic, Heat Exchangers Basics design applications, In-Tech Rijeka, (2012) [19] A. Kraus, A. Aziz, J. Welty, Extended surace heat transer, A Willey Inter science Publication, [20] F. C. McQuiston, D. R. Tree, Optimum space envelopes o the inned tube heat transer surace, ASHRAE Transactions, Vol. 78, Part 2, (1972) [21] G. H. Hewitt, G. L. Shires, T. R. Bott, Process Heat Transer, CRC Press, [22] R. W. Serth, Process heat transer: principles and applications, Elsevier USA, [23] T. Kuppan, Heat exchanger design handbook, Marcel Dekker USA, [24] O. James, J. O. Maloney, Perry s chemical engineers handbook. Mc Graw- Hill, USA,2008.