OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 7 5 on uneven surfaces [16]. Qu et al. [17] demonstrated that internal capillary tube roughness significantly improves startup and operational stability in micro-pulsating heat pipes (PHPs). Surface tension, fluid viscosity, and wall roughness all affect flow resistance, which limits stable pulse operation in fixeddiameter PHPs [18]. Singh et al. [19] investigated ordered roughness and pulsed flow in microchannels using 2D simulations. Due to enhanced vortex activity, pulsation raised the Nusselt number (Nu) by up to 32.76% regardless of roughness. They also found that the optimal pulsation frequency varies with hydraulic diameter and that rough walls result in larger pressure decreases, even with heat transfer (HT) improvements. Wu and Cheng [20] discovered Nu fluctuations in shape-variable trapezoidal silicon microchannels. In waterfilled minichannels, Lin et al. [21] discovered that roughness heights between 18 and 96 μm improved HT. Lu et al. [22] confirmed that roughness raised flow resistance and Nu in laminar microchannel flows. Croce et al. [23] showed that roughness shape has a greater effect on pressure drop than Nu. Despite extensive research on pulsating flow dynamics, heat transfer (HT) mechanisms remain incompletely understood [24–32]. Analytical and numerical investigations in laminar flow [33–37] demonstrate localized HT effects, with pulsation-induced Nu fluctuations being dominant near the pipe entrance and decreasing downstream. Despite extensive research on pulsating flow dynamics, the underlying heat transfer (HT) mechanisms remain incompletely understood [24–32]. Analytical and numerical investigations in laminar flow regimes [33–37] demonstrate localized HT effects, wherein pulsation-induced Nu fluctuations are most pronounced near the pipe entrance and diminish in the downstream direction. Methodology Fig. 1 shows the experimental setup. A copper pipe, 400 mm in length and 28 mm in diameter, serves as the test section. Flexible joints hold it in place at both ends. Four K-type thermocouples are embedded in axial grooves on the outer surface of the pipe and connected to a multichannel recorder via a multipoint switch to record temperature measurements. A 400 mm long nickel-chromium heater (resistivity = = 15.5 Ω/m) provides uniform heat input. Airflow is provided by a 1.5 HP centrifugal blower (800 CFM), selected for its ability to maintain turbulent flow conditions. An electrically operated solenoid valve introduces flow pulsations. Operational boundaries are influenced by static pressure, temperature rise, and Reynolds number (Re). A flow control valve (¾” brass valve, 12V DC, 1.5 A/18 W, orifice size 25 mm, normally closed, stainless steel components), as shown in Fig. 2, is used to regulate airflow with a sub-second response time. The valve allows for adjustment of the pulsation mechanism to provide the required amplitude and frequency of pulsation. Fig. 1. Experimental set up Fig. 2. Flow control valve
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