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  • Finding friction factor for low temperature helium flow through serrated type plate-fin heat exchanger using CFD

    B V Shah 1, A K Sahu2, N Mamgain 2, S V Jain 3, P Shrama4 1Bhakti Consultants, Gandhinagar, India

    2Institute for plasma research, Bhat Village, Gandhinagar. Gujarat, India 3 Institute of Technology, Nirma University, Ahmedabad, Gujarat

    4 Indus Institute of Engineering and Technology, Rancharda, Kalol , Gujarat, India *E-mail : [email protected] , [email protected]

    Abstract: Serrated type plate-fin heat exchangers are good for high heat transfer coefficients due to high turbulence. But the serrated type gives larger pressure drop than that of other types. These can be used suitably for helium plant application, where high thermal effectiveness is important, but at the design phase it should be ensured that pressure drop is not high and it should be about few tens of mbar. The present analysis has been done to find friction factor and pressure drop for a segment of plate-fin exchanger of helium plant considering empirical co-relations of Manglik and Bergles and computational fluid

    dynamic (CFD) software. This heat exchanger is designed for 2-stream counter-flow configuration with serrated fins and for operation with temperature range between 7 to 4.5 K, at pressure 4 bar for hot helium stream and 1.2 bar for cold helium stream. Serrated fins of thickness 0.2 mm, has been used.

    Calculated pressure drop, based on empirical co-relations, for this heat exchanger for both streams is less than 10 mbar. The pressure drop, found from CFD analysis is higher than that found from empirical co-relations. This paper will discuss in detail the method and results of CFD & co-relation based analysis.




    For serrated fins, friction factor, f, is given by MB [1] correlation as below for 120 12.5 mollion, the variation in pressure drop is less

    than 5 %.

    Hence, further analysis is done with element size corresponding to this meshing and it has 0.16 mm

    as minimum element dimension and 0.32 mm as maximum dimension.

    In the present study, analysis is done with different turbulence models (Figure 5 ).

    It shows (with a model having length of 38.8 mm and width 3.38 mm) that the results obtained

    with standard k- model follows the pattern of co-relation results.

    Hence, standard k- model is used for further analysis.




    Table 2: HE8 designed parameters [5]

    (a) (b)

    (a) (b)

    Table 5: Friction factor comparison

    Domains chosen for modelling and analysis with constant helium property

    Modelling and Grid generation

    For useful comparison of friction factor estimated through MB co-relation and CFD analysis,

    temperature range of helium fluid is taken as 100 to 110 K

    Other parameters kept constant as in HE8.

    Further as the no. of elements will be large, a smaller segment which can represent the HE8 is

    considered for modelling and analysis.

    To find the appropriate element size, suitable for this analysis, grid independence test is also done

    (figure 4).

    CFD analysis with different solution options are carried out with constant Helium properties to

    select the best solution technique. With these different sizes of HE segments are modelled and

    analysed for 100 to 110 K temperature range.

    Later, the CFD analysis is carried out with non-linear variations of Helium properties in the

    temperature range of 4.5 to 7 K for HE8

    Here, s is spacing between fins, h is fin height, l is fin serration length and t is fin thickness

    Total drop in pressure for hot or cold stream, is expressed as,

    = + , where =42

    2 , Here, f is fanning friction

    factor, density of fluid, = (not considered here)

    = 0.6522;0.5403




    1 + 5.269x10;51.340



    ;1.055 0.1

    [6] Kays W M and London A L ,Compact Heat Exchangers, 2nd ed., McGraw-Hill, New

    York,1964.Joshi, H.M., and Webb, R.L., Prediction of Heat Transfer and Friction in the

    Offset Strip Fin Array, International Journal of Heat and Mass Transfer. 30, 69 84, 1987

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