Numerical investigation on the flow and heat transfer characteristics of waxy crude oil during the tubular heating

Abstract

In industrial practice, tubular heating is known as an effective technology to ensure the security storage of waxy crude oil in the tank. Insights of the thermal and flow behaviors during the tubular heating is a key aspect to optimize the strategy of this heating technology, in order to reduce the cost. In this study the physical and mathematical models representing the heat transfer and flow of waxy crude oil during tubular heating are established. The additional specific heat capacity and momentum source terms methods are employed to address the changing physical properties related to the paraffin crystallization and dissolution. The FVM, PISO and fully implicit first order temporal differentiation algorithms are used to perform the numerical simulation. Our outcomes show that the thermal process during tubular heating can be divided into four phases: ①Local thermal response phase, ②Thermal diffusion phase, ③Global thermal response phase, ④Elimination of gelled oil phase. The initial thermal influence range is inhibited by the thermal condition and solid-like gelled waxy crude oil formed after cooling. The plume flow takes great effects of transferring the heat from the tubes to crude oil and accelerating the phase transition, and finally facilitates the oil temperature uniform increasing in most parts of the tank. Whereas the gelled crude oils in the corner between the sidewall and base wall have the strongest solid-like character and lowest temperature. A low temperature layer keeps covering the base wall. At the top wall, the center region between the sidewall and center of the tank is long time occupied by gelled oil. The proportion of solid-like gelled crude oil keeps decreasing with a small slope value following a linear relationship with heating time. A pronounced phase change of waxy crude oil from the transition state into the pure liquid is observed. The minimum temperature value decreases in the initial heating period, and then steeply increases to an approximate plateau value. The fluctuations of the heating powers are observed indicating the transient thermal convections around the tubes. The heat loss power is lowest at the base wall and nearly unchanged. The heat loss power at top wall and sidewall are significantly influenced by heating tubes and depend on the wall temperature.