When the boiling phenomenon
had occurred and the temperatures have reached almost a steady state, the values of the liquid flow rate or the heat flux of the power source were varied and the same procedure was repeated. For each fixed experimental condition, the test section was heated and the temperatures were monitored continually. Experiments were performed with deionized water and silver-water nanofluids. Experimental results presented in this paper were treated only in the steady state when the wall temperatures become approximately constant with time. The temperatures fluctuation is about ±0.1°C. The local heat transfer coefficient of each axial check details location along the channel length is given as follows: (1) where q channel, x is the local heat flux estimated by taking PF-6463922 in vivo into account the local heat loss, T s,x is the local surface temperature, T f is the fluid bulk mean temperature, and x is the axial coordinate parallel to the flow’s direction. The local heat flux
is calculated depending on Fourier’s law: (2) where λ w(=389 W/mK) is the thermal conductivity of the copper wall, T 1,x and T 2,x are the temperatures measured inside the copper plate, Δy is the space between thermocouples locations inside the wall (see Figure 4b). The vapor quality is defined as the ratio of the local vapor Wortmannin cell line mass flow rate to the total mass flow rate . Applying the energy balance equation between the inlet and the outlet of each subsection yields (3) where q channel,x is the local heat else flux along the flow direction, h fg is the heat of vaporization, W channel is the channel width, T sat is the working fluid saturation temperature, T f is the working
fluid inlet temperature, C pl is the liquid working fluid specific heat capacity, and is the single channel mass flow rate determined from the assumption that the total mass flow rate is uniformly distributed in the minichannels, (4) where G is the total mass flux measured during experiments, H channel is the channel height, W channel is the channel width, and N channel is the number of channels. A Denver Instrument flow meter (Bohemia, NY, USA) is used to measure the mass flow rate of the working fluid with an uncertainty of 1.3%. Furthermore, microthermocouples calibration is carried out by comparing the temperatures measured by each microthermocouple to those measured by a high-precision sensor probe (±0.03°C). The uncertainties in heat flux, heat transfer coefficient, vapor quality, and mass flux (Equations 1, 2, 3, and 4) were evaluated using the method of Kline and McClintock . For example, the uncertainty of the heat flux was evaluated by the following: (5) where q is the heat flux along the flow direction, λ the thermal conductivity of the copper plate, T is the temperature measured inside the copper plate for different levels, Δy is the space between thermocouples locations inside the copper plate.