Comparison of h for all fluids between TMMHP and SMMHP
Figure 19 (a), (b) and (c) show the values of heat transfer coefficient of different fluids at inclinations of 0o, 45o and 90o. In each case methanol possesses the highest ‘h’ among all the fluids, and the highest value is attained at inclination 45o. Thus in respect to h , methanol is the most valuable working fluid out of the four for TMMHP. However, the sequence of ‘h’ values for the four fluids remains the same at all three orientations.
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Figure 19 (a). Convec. coeff. of fluids vs. heat input in TMMHPFigure 20 (a). Eff. c onvec.coeff.of fluids vs.heat inpu tinTMMHP
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Figure 19 (b). Convec. coeff. of fluids vs. heat input in TMMHPFigure 20 (b). Eff. c onvec.coeff.of fluids vs.heat inpu tinTMMHP [CHART] [CHART]
Figure 19 (c). Convec. coeff. of fluids vs. heat input in TMMHPFigure 20 (c). Eff. c onvec.coeff.of fluids vs.heat inpu tinTMMHP
In Figure 20, the sequential rise of h of all four fluids in TMMHP at 45o is shown. Methanol gains the highest value of ‘h’ whereas ethanol gives the lowest. Since the surface temperature of the TMMHP is depended on the heat input and heat rejection at the evaporator and condenser respectively, such extreme values of ‘h’ become dependent only on the internal fluids’ overall thermophysical properties. According to Newton’s law of cooling,h of a system with constant heat input and surface area gets the highest value for the smallest terminal temperature difference within the heat pipe and vice versa. This correlation can be authenticated by comparing Figure 16 and Figure 20 where methanol achieves the highest value of h . Consequently, at a small terminal temperature difference, the sharp decrease of pressure gradient leads to rapid condensation at the condenser port to increase the h value. As the vapor becomes liquid at the condenser, the density of the fluid therein goes many folds high. However, the h keeps no direct relationship with the density alone which reflects in both Figure 19 and Figure 20. Rather it is found that h is compositely related with the fluid’s density, pressure drop and heat input. This relationship can be expressed by\(\mathbf{\ }h=f(\rho\left(p\left(q\right)\right))\). Then the authors have developed the dimensionless correlations from this relationship presented later.
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Figure 21(a). Comparison of h/hef between TMMHP and SMMHP [6] Figure 21(b). Comparison of h/hef between TMMHP and SMMHP[6]
In Figure 21(a-b), all the fluids’ dimensionless heat transfer coefficients are shown including the water’sh/heff at single-metal micro heat pipe. The maximum value of methanol is seen because of its lowest boiling point (66 oC) which allows it to complete the boiling-condensation cycle faster than the other three liquids. However, comparing the water’s h/heff value at TMMHP is twice as high as that of at SMMHP [6]. Thus, the two different thermal conductivities at the two ports of the TMMHP initiate the quicker heat removal than that of the SMMHP, hence improves the hso greatly.
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Figure 22 (a) Thermal Resistance vs. Heat input at TMMHPFigure 22 (b) Thermal Resistance vs. Heat input at TMMHP
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Figure 22 (c) Thermal Resistance vs. Heat input at TMMHPFigure 22 (d) Thermal Resistance vs. Heat input at TMMHP
In Figure 22 (a-d), effective thermal resistances are shown for all the working fluids used in the TMMHP. It is obvious that the orientation of the heat pipe plays an important role on maintaining the order of thermal resistance for particular fluid. However, except water three liquids are keeping almost the same lower trend of thermal resistance with the increasing heat input. This difference is occurred because of the two different kinds of fluids where water is covalent compound and other three are hydrocarbons. Having such difference in chemical structure, their thermophysical properties (i.e. density, specific heat etc.) also go different resulting thermal resistances. In all four cases, it is seen that like the heat transfer coefficient, thermal resistance also takes a turn as the heat input is increased to a moderately high value (i.e. 10W). Methanol is found to be of the lowest thermal resistance as 0.43 o C/Watt as it is in congruence with the highest value of h in comparison.
A comparison of thermal performance between single-metal (SMMHP) [6] and two-metal micro heat pipe (TMMHP) observed in this study is given in Table 2.
Table 2. Comparison of thermal performance between SMMHP [6] and TMMHP