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Heat Transfer Theoretical Analysis Experimental Investigations and Industrial Systems Part 15

Tham khảo tài liệu 'heat transfer theoretical analysis experimental investigations and industrial systems part 15', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 550 Heat Transfer - Theoretical Analysis Experimental Investigations and Industrial Systems maximum. The shape of the distribution depends only on KiSi where Ki Ợ 2a a is thermal diffusivity of the insulating layer. For lower frequencies KiSi 1 the distribution can be assumed linear while for higher frequencies Kisi 1 the temperature fluctuates only in the vicinity of the foil y ỗj 1 KiSi . Fig. 2. Instantaneous temperature distribution in the insulating layer at rot n 2 and Tw Tc Introduce the effective thickness of the insulating layer 8j f the temperature of which fluctuates with the thin foil õì f 0.5ỔÍ KiSi 1 19 di f 0.5 Ki KiSi 1 . 20 The heat capacity of this region works as an additional heat capacity that deteriorates the frequency response. Thus the effective time constant considering the heat losses can be defined as T K cpS CiPi Si f ht . 21 ht Tw - T0 Here ht is total heat transfer coefficient from the thin foil including the effects of conduction and radiation. Then the cut-off frequency is defined as follows 1 fc . 22 2nT We introduce the following non-dimensional frequency and non-dimensional amplitude of the temperature fluctuation f f fc 23 ATw f ATw f ht Tw - T 0 Ah 24 Spatio-Temporal Measurement of Convective Heat Transfer Using Infrared Thermography 551 Here ATw f includes the factor ht Ah to extend the value of ATw f to unity at the lower frequency in the absence of conductive or radiative heat losses see Fig. 3 . 0.1 1 10 100 f f fc Fig. 3. Relation between non-dimensional frequency f and non-dimensional fluctuating amplitude ATw f Next we attempt to obtain the relation between f and aTw . The fluctuating amplitude of the surface temperature ATw f can be determined by solving the heat conduction equations of Eq. 1 and 7 by the finite difference method assuming a uniform temperature in the x-z plane. Figure 3 plots the relation of ATw f versus f for practical conditions see sections 5 and 6 . The thin foil is a titanium foil 2 pm thick cpỗ 4.7