Abstract
One of the most commonly used methods of heat transfer enhancement is flow turbulization. This effect can be achieved, e.g., by placing special turbulizing elements into the channel. In this paper, the effects of ball turbulators (BTs) on the heat transfer and fluid friction characteristics in a circular tube are investigated through numerical simulation. The Reynolds number (Re) is in the range of 5000–35,000 under a condition of uniform heat-flux. BTs with different diameter ratios (e.g., 0.5, 0.75, and 1) and spacer lengths (40, 51.77, and 62.5 mm) are inserted in the circular tubes. The results show that the heat transfer rates in the tube equipped with BTs are around 1.26–2.01 times that of those in the plain tube. The BTs with a ball diameter ratio of one provide higher friction factors than 0.75 and 0.5 by about 34.6–46.2% and 51.1–63.4%, respectively. A smaller ball diameter ratio is more able to decrease the friction factor. The performance evaluation criterion (PEC) data indicate that the use of a smaller ball diameter ratio (BDR) and a smaller spacer length are preferred. The results also reveal that BTs with a larger diameter ratio and a smaller spacer length yield the highest heat transfer rate as well as the largest pressure loss. Compared with the plain tube, the fluid flow velocity near the tube wall is significantly improved when BTs are used at the same Reynolds number.
Highlights
Heat exchangers are widely used in many industrial fields including power generation, the petrochemical and metallurgical industries, etc
Experiment coil-wire inserts twisted tape; pressure drop; full-length twisted tap and pressure dropthe improved structure could reduce the pressure by 40%
The Nusselt number increases and friction factor decreases as the Reynolds number increases; smaller ball diameter
Summary
Heat exchangers are widely used in many industrial fields including power generation, the petrochemical and metallurgical industries, etc. In order to reduce energy consumption and prevent thermal failure of the target devices and materials, researchers have devoted themselves to developing techniques for heat transfer enhancement for decades. Thermal augmentation techniques can be divided into two categories, i.e., active and passive [1,2]. The passive techniques with the tube inserts are most frequently used without direct application of external power. Many papers have reported on the passive techniques with the tube inserts. Twisted tape is the most commonly used solution because of its simple configuration and steady performance [3,4]. The critical mechanism of heat transfer enhancement of twisted tape is its ability to induce swirl flow and increase the turbulence intensity close to the tube wall, which can promote mixed flow from the near-wall and central regions
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