Flow induced vibrations (FIVs) in heat exchangers, tube bundles, rod bundles etc., have been a major concern in nuclear, petrochemical and power generation industries [1–4]. Nuclear fuel pellets are stacked in clad tubes which are positioned either horizontally or vertically. The coolant flows through the gaps (called sub-channels) between the fuel rods and extracts heat along the length of the sub-assembly. As the coolant extracts heat, unbalanced forces and moments on the rod surfaces are often generated due to variable flow paths through the bundle configuration, which is further aided by the coolant phase change. Span-wise stream of coolant past a cylindrical rod surface is a source of oscillatory force as it may result in flow separation, eddy formation and shedding, which are all sources of axial flow-induced vibrations. A number of studies are dedicated to the axial FIV caused by single-phase. Santis and Shams [5] performed numerical simulation for a two rod system (on x - axis) and studied the effect of vibration of one rod on the other. They concluded that the solid rods vibrate in both x and y-directions, however, due to the effect of the neighboring rod the vibration in the x-direction is more intense. Similarly, Hofstede et al. [6] simulated the predisplaced single rod and compared the predictions against the experimental displacement of the rod. Based on their simulations, it is concluded that with increase in fluid velocity there is a decrease in the frequency of vibration of the structure. Although, several studies on single phase axial FIV are reported, not many attempts have been made to study the influence of axial FIV on the thermal characteristics in a fuel rod bundle. To this end, analysis of heat transport from the heated surfaces of heat exchangers, heated fuel rods etc., is very much essential to identify the operating conditions that would lead to axial FIV in a fuel rod bundle.
As the coolant extracts heat, the coolant forms a dispersed phase in the flow domain, which is known as flow boiling. Although phase change enables higher heat transfer rates in the form of latent heat, higher the vapour phase volume fraction near the heated surface, lower the heat transfer rate. This may be primarily attributed to the poor heat transfer characteristics of vapour phase. Therefore, the advantage of phase change can be exploited only by achieving controlled phase change phenomenon, which aims at maintaining the bulk liquid temperature below the coolant saturation temperature - termed as subcooled flow boiling. In this regime, the vapour bubbles are formed at the preferred nucleation sites on the heated surface when the wall superheat is adequate to trigger the coolant phase change. The formed vapour bubbles grow to a maximum size, called bubble departure diameter, to depart from the respective nucleation sites on the heated surface. The bubble ebullition
cycle plays a significant role in determining the rate of heat transfer from the heated surface.