Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN

Size: px

Start display at page:

Download "Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN"

Oliver Ramsey
5 years ago
Views:

Transactions on Engineering Sciences vol 12, 1996 WIT Press, www.witpress.com, ISSN 1743-3533 Flow phenomena of laminar split-oil flow in a curved horizontal channel M. Deli6,* P. Skerget," F.

1 Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN Flow phenomena of laminar split-oil flow in a curved horizontal channel M. Deli6,* P. Skerget," F. Irene* ^University ofmaribor, Faculty of Mechanical Engineering, Smetnova 17, SI Maribor, Slovenia * University of Ljubljana, Faculty of Mechanical Engineering, Askerceva 6, Ljubljana, Slovenia ABSTRACT A curved, rectangular channel fed with lube oil was installed in the cylinder wall of an air cooled internal combustion engine. Inlet oil-flow is initially split into two asymmetrically curved branches. Different channel geometrical aspect ratios were used to determine flow pattern especially in the entry region of the channel. Results have shown pronunced influence of oil viscosity on the reverse flow in both channel branches. INTRODUCTION Due to different thermal loads of the engine cylinder with peak value in the vicinity of the exhaust channel, there is pronouced asymmetric circumferential temperature distribution in the cylinder wall. This temperature ovalty can be minimized by controled intensity of the local cooling, which can be obtained by introducing a curved square cross-section oil channel in the upper, thermally most loaded part of an air cooled engine cylinder. The cooling oil jet is initially split into two asymmetrical branch flows which leave the channel through a common outlet. Partial oil flows are not identical; the larger portion flows through the region of pronouced local cylinder temperatures and takes away more heat. The smaller oil-flow cools down cooler part of the cylinder wall. For correct realisation of cooling it is necessary to know the local thermo-hydraulic conditions in the channel. Conditions in the channel can be defined numerically or experimentally. The problem was solved numerically, because the numerical solution is economicaly more favourable. With the numerical solution it is also easier to change boundary conditions and introduce geometrical changes. Modification of the channel thickness represents a significant influence on the functionality. Because of that the thermo-hydraulic conditions in the

Transactions on Engineering Sciences vol 12, 1996 WIT Press, www.witpress.com, ISSN 1743-3533 116 Advanced Computational Methods in Heat Transfer channel essentially change.

2 Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN Advanced Computational Methods in Heat Transfer channel essentially change. Therefore the calculations for three different channel croos sections were made. We also investigated influence of mass flow on heat transfer. The FEM mesh of an engine cylinder and position of the oil channel for different version are shown in figure 1 [4,5]. DOH5 Figure 1: FEM mesh and position of the channels in the cylinder PROBLEM DEFINITION In the calculations the comparison between three different channel cross sections were made: on the inlet mass flow, which corresponds to volume flow of 5//mm is given, on outflow reference pressure is prescribed, and on the walls the velocity vanishes. On the pipes adiabatic boundary conditions are given, and on all other solid walls constant surface temperature T = IIQ C is given. Temperature of the inflow fluid is given at 100 C. In the calculations where the influence of oil mass flow was analysed, on the inflow the mass flow which corespond to volume flow of 3, 5 and 7 //ram are given. At the iner wall constant surface temperature T = 150*C, and for the outer wall constant surface temperature T = 145^C is given. On the all other walls adiabatic boundary conditions are given. Temperature of the inflow fluid is given at 97 C. To solve the problem, a TASCflow 2.4 computer program was used. The code is based on finite volume method, solving differential equations of transport phenomena in incompressible fluid flow. This equations are basics conservation balances of mass, momentum and energy. The material properties of the fluid (oil) such as specific heat, thermal conductivity and density are given by the following expressions: [6],

3 Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN Advanced Computational Methods in Heat Transfer c, (T) = T -, [kg it J A(T) = T [ ], L TTLJ\ J p (T) = ( (T - 20)) ^. (1) (2) (3) In the calculations where the different channel cross sections were compared, the following expression for dynamic viscosity was used: = ( (4) and for calculations where the influence of mass flow was investigated the oil viscosity was given with 7?(T) = ( T* T ) 10"' [Pa s] (5) In all expressions temperature T is given in *C. To test the convergence of the numerical results more meshes were applied with the finest one of nodes for all three cases. 1 4>io outflow q = o 47 [mm] di DOH3 II 139 DOH4 139 DOH5 II 139 do h M Figure 2: Geometry of engine cylinder oil channels and boundary conditions

4 Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN Advanced Computational Methods in Heat Transfer RESULTS The numerical results obtained for heat transfer with empirical expressions for Nusselt number (Nu) [2] were compared. For average Nu number, the expression: (^]RePr Nu = ^ r (6) was used, and for local Nu number: i The constant 3.66 in equation (6) is changed to consider the relation between the edges of the channel cross section and it is for relation 14 : 3.2 (DOH3), for relation 18 : 2 (DOHS), and for relation 37 : 1 (DOH4). In both expressions hydraulic diameter (<4 = 4A/O) is considered, where A and O are area and circumference of the channel crosssection. Re (Re v dhp/rj) and Pr (Pr c^7]/x) are Reynolds and Prandtl numbers. Influence of the curvature was considered with relations: (7), NU;R = Nux6n, (8) where e^ is correction factor for the relative curvature, given by expression: e«= ^, (9) ri where R is the radius of the curvature. Numerical results were treated with expressions where and - adh otxdh,,_\ Nu = and Nux = r, (10) A A 1 ri a = - I a^dx (11) t Jo Temperature difference is given by equation ATm = T,-Tm, (13) where T, is a wall temperature and T^ is the mean temperature at the cross section. The mean inflow (T^t,) and outflow (7^) temperature from the channel was determined by Tm - ^"2 (14) me.

Different channel cross-sections computational Methods in Heat Transfer 119 Transactions on Engineering Sciences vol 12, 1996 WIT Press, www.witpress.

5 Different channel cross-sections computational Methods in Heat Transfer 119 Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN Figure 3 presents streamlines in the channel DOH5. At given geometry, material properties and boundary conditions the procentual parts of mass flows and Re number throught the branches are presented in table 1. II II "». [%] DOH3 DOH4 DOH mi [%] Re, Rei Table 1: Parts of mass flow and Re number in the both branches Figure 3: Streamlines of the channel oil flow (DOH5) Figure 4: Velocity distribution in inlet region at 1/4, 2/4 and 3/4 height of the channel (DOH5)

6 Trig,riii,R^s Transactions and on Engineering Rei are Sciences the mass vol 12, flows 1996 and WIT Re Press, numbers through ISSN the shorter and longer branch. On Figure 5, the influence of impinging jet and channel contraction from longitudional section is clearly evident. Because of the impinging oil jet, circulation is impeded in longitudinal and cross-section, which additionaly increases heat flux throught the wall of the channel. In the Figures 6 and 7, the average cross heat transfer throught both branches of the channel (DOH5) is shown. numerical results equation (6) + influence of curvature (8) numericni equation (7) + influence of curvature (8) ANGLE Figure 5: Nu and Nu^ along the shorter branch of the channel DOH5 * Nu Nu numerical results equation (6) -f influence of curvature (8) numericni equation (7) + influence of curvature (8) _ '* ANGLE Figure 6: Nu and Nu^ along the longer branch of the channel DOHb

7 Advanced Computational Methods in Heat Transfer 121 Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN It is evident that the numerical results are in good agreement with the results obtained from equations (6) and (7), which were selected for comparison with numerical results. The greatest values are on the inflows in the branches and then the heat transfer decreases toward the outlet outflow, where because of extension of the channel (see Fig. 2) and with the thinning of the boundary layer - heat transfer instantly increases. Through the shorter branch of the channel, Nu number has higher values because of the greater mass flow and the shorter branch of channel (boundary layer development), and with that the cooling is more intensive on the thermically more loaded part of the engine cylinder. On contractions and extensions of the channel, the velocity boundary layer becomes thinner. This is the reason for the strong increase of heat fluxes. Figure 7 presents a comparison of average heat fluxes at circumference for different cross section channel. Higher heat transfer in the thinner channel is the consequence of larger velocity gradients, which are conditioned by channel thickness (lover edge of cross section). In the vicinity of inflow and outflow the channel is extended (sharp change of the geometry), therefore local maxima of heat flux occour on contractions and extension of the channel. On the graph is evident influence of inflow jet from curve convexity on the inflow region. x channel DOH3 - channel DOH4 4- channel DOH ANGLE Figure 7: Average heat flux along the channel for different cross sections

8 Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN Advanced Computational Methods in Heat Transfer Different mass flows Cooling can be also controled with changing the mass flow. The boundary conditions used for this calculations are cloaser to the real boundary conditions, then in the previous calculations. Dynamic viscosity is given with expresion (5) (motor oil). This expresion give the lower value of dynamic viscosity then equation 4, therefor the Re number in this calculations is higher. The geometry of the channel corresponds with the geometry of the channel DOH3 (14 x 3.2mm) from previus calculation and for the computation same mesh was used. At the higher flow rates, the velocityfieldis more disturbed and therefore longer way for developing velocity end temperature profiles is needed. The procentual parts of mass flows trought both branches are presented in the followig table. m/ Re. V = 3 //rain V = 5 //rain V = 7 //rain , Table 2: Parts of mass flow and Re number in the both branches From table is evident, that at higher mass flows, fluid is not distributed between the channel branches, becouse one part of fluid flow circulated in the channel,therefor flow change direction in longer branch. From the Figure 8 it is clean that the flow in longer branch is completly stagnant for the inflow volume flow of //rain. On Figure 8 the part of mass flow throught the longer branch is shown. ;[%) f\o " Icy -21 change of fl< DW direction in longer branche *«.. "... *#... *** V [1/min] Figure 8: Part of mass flow throught the longer branch Figure 9 represents the inlet region at 1/4, 2/4 and 3/4 height of the channel. The velocity gradients near the inner wall are larger then in the previous

9 Advanced Computational Methods in Heat Transfer 123 Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN example (Fig. 4), because of lower dynamic viscosity. Also the horizontal circulation is more intensive and propagates further in short branch of the channel. Figure 9: Velocity distribution in inlet region at 1/4, 2/4 and 3/4 height of the channel (V 5 //mm) On the figures 10 and 11 the average heat fluxes along the inner and outer wall for all three channels are shown. It is resonable that the higher values for heat fluxes are the consequece of higher mass flow. The maxima of heat fluxes are on the place of impinging oil jet and on the contractions of the channel. - V = 7 //ram + V = 5 //ram x V = 3 //ram 60' H ANGLE Figure 10: Average heat flux along the inner wall of the channel

10 124 Advanced Computational Methods in Heat Transfer Transactions on Engineering Sciences vol 12, 1996 WIT Press, ISSN V = 7 //ram f V = 5 //ram x V = 3 //ram ANGLE Figure 11: Average heat flux along the outer wall of the channel CONCLUSIONS With additional oil cooling temperature loads and their circumferential ovalty on diesel engine cylinder can be reduced. Numerical results are in good agreement with empirical expressions. In the inlet and outlet region fluid is mixed due to the influence of inflow jet. Developing of velocity and thermal boundary layer increase intensity of heat transfer. Maximum values of heat fluxes are on the place of impinging oil jet and on the contractions of the channel (inlet). REFERENCES [1] M. Delic, P. Skerget, I. ZaganNumerical and experimental validation of thermo-hydraulic conditions in the narrow channels, Advanced Computational Methods in Heat Transfer III, Southampton 1994 [2] S. P. Sukhatme:Correlations in Single-Phase Convection Heat Transfer, Heat Transfer Equipment Design, Hemisphere, 1988 [3] TASCflow User Documentation - Version 2.4, March 1995 [4] F. TrencrAnaliza temperaturnega stanja na valju zracno hlajenega motorja, Ph.D., Ljubljana, 1991 [5] F. TrencrAnalysis of Combined Air-Oil Cooling Effectiveness of Diesel Engine Cylinders, Strojniski vestnik, Ljubljana, 1995 [6] R. Vicic:Studij toplotnih tokov na modelu oljnega kanala v segmentu valja zracno hlajenega motorja, M.Sc, Ljubljana, 1992

Numerical analysis of fluid flow and heat transfer in 2D sinusoidal wavy channel

Numerical analysis of fluid flow and heat transfer in 2D sinusoidal wavy channel Arunanshu Chakravarty 1* 1 CTU in Prague, Faculty of Mechanical Engineering, Department of Process Engineering,Technická