2 2.4 The value of the measured specific heat C p of the base fluid as well as the nanofluids are comparable (C p  ≈ 2.5 J/g K). It is thus clear that the enhancement of the effusivity in both the nanofluids is arising primarily due to the enhancement of the thermal conductivity κ. To make an independent check on the enhancement of the thermal conductivity, we used the measured frequency dependence of the thermal oscillation

δT 2ω . Equation 4 gives a limiting low-temperature slope for δT 2ω wrt the frequency (log f) that is proportional to κ −1. Using this information, we obtain the relative enhancement of the thermal conductivity wrt the base fluid ethanol. The data for both the nanofluids are shown in Table 1. It can be seen that this also gives this website nearly the same value for enhancement (within 15% to 20%), which confirms that there is indeed an enhancement in κ in the nanofluids. It is gratifying that the analysis from both the parameters δT 2ω and gives similar results. It can be seen from Table 1 that the enhancement κ for the bare ZnO nanofluid is significantly larger than that

seen in the PVP-stabilized ZnO nanofluid. This gives us the first important result that there is indeed a significant reduction in the effusivity MS 275 and thermal conductivity on stabilizing the ZnO nanofluid with stabilizer that inhibits the local aggregation significantly, which in turn leads to its long-term stability. This observation establishes a direct connection between the enhancement of κ and the local

aggregate formation. The frequency dependence of the enhancement and its analysis The enhancement of the effusivity in nanofluids has a frequency dependence as shown in Figure 3, where the enhancement decreased at higher frequency, and for f > 30 Hz, the values of C p κ for both the nanofluids approach that of the Tyrosine-protein kinase BLK base fluid ethanol. This frequency dependence of the effusivity for bare ZnO nanofluid (without PVP) has been reported elsewhere [15]. It was proposed that the frequency dependence can arise from dynamic local aggregation. In this paper, we explore the proposed PRN1371 chemical structure hypothesis whether the frequency dependence indeed has a connection to the local aggregation. At low frequency (f ≤ 10 Hz), the enhancement is large, and it reaches a frequency-independent value. The decrease in the effusivity at higher frequency in both the nanofluids can be fitted by the low-pass filter relation: (5) The corner frequency f c and the order of the filter n can be obtained from the fit to the data. For the ZnO nanofluid without PVP, the data can be fitted by the first-order filter function (n = 1). For fluid with PVP, we got a different higher order value, which is n = 5. In Figure 4, we show the fit of the data to Equation 5. The data for both the nanofluids are shown. Figure 4 Low-pass filter response fit for ZnO nanofluids and ZnO-PVP nanofluid. The data are summarized in Table 2.