Heat Transfer to Mercury Flowing In-Line Through an Unbaffled Rod Bundle: Effect of Rod Displacement on Local Surface Temperature and Local Heat Flux

An experimental study of heat transfer to mercury flowing longitudinally through an unbaffled rod bundle was carried out. The purpose was to determine the effect of lateral displacement of a rod on local heat transfer behavior. In a previous paper, the effects of extent and direction of displacement on the rod-average heat transfer coefficient were presented for the displaced rod, on that (or those) toward which it was displaced, and on that (or those) from which it was displaced. Here, the effects of extent and direction of displacement on the peripherally local heating surface temperature, local heat flux, local heat transfer coefficient, and local surface temperature fluctuations are presented for the displaced rod. The test bundle had a P/D ratio of 1.75, and the rods were special electrical heaters. It was found that rod displacement can cause a large circumferential variation in its local heat transfer characteristics. Aside from the P/D ratio, the independent parameters affecting these characteristics are circumferential angle (θ), relative cladding thickness [(r₂ − r₁)/r₂], relative cladding conductivity (k_w/k_f), and flow rate (Pe). It was found that displacement of a rod can produce circumferential variations in its surface temperature comparable to the average temperature drop from the heating surface to the coolant stream. For a given displacement, this variation increases as average heat flux increases and as (r₂ − r₁)/r₂, k_w/k_f, and Pe decrease; changes in have the greatest effect, and those in (r₂ − r₁)/r₂ and k_w/k_f, the least. For a given displacement and flow rate, the greater the surface temperature variation, the less will be the circumferential variation in the local heat flux. Thus, as either cladding thickness or conductivity increase, the variation in the local heat transfer coefficient (and therefore the average) remains about the same. It was found that, as a rod is displaced from its symmetrical position, the local heat transfer coefficients surprisingly decrease at all circumferential points, which partly explains why the rod-average heat transfer coefficient is highly adversely affected by lateral rod displacement. This is only true for liquid-metal coolants.