Articles written in Sadhana

    • Mixed convection in a baffled grooved channel

      Aayush K Sharma Pallab S Mahapatra Nirmal K Manna Koushik Ghosh

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      In the present numerical work, flow structure and heat transfer characteristics are investigated in a baffled grooved channel, differentially heated from the sides. The baffle is placed vertically downward from the top wall of grooved channel geometry, with the motive of diverting outside forced flow towards the inside of the square cavity. In-house CFD code based on finite volume method has been used to solve the 2D equations of continuity, momentum and energy. The effect of change in baffle position and height is investigated in the range of Richardson numbers 0.1 to 10. For the present study, external flow from both left and right of the grooved channel are considered. A remarkable enhancement of heat transfer is observed in presence of baffle. The study has also pointed out that for optimal performance, the position and height of the baffle need to be adjusted depending on the direction of external flow.

    • Pressure dependence of dryout in a heat-generating porous debris bed


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      The primary objective of this work is to numerically study the effect of system pressure on the occurrence of dryout phenomena in heap-shaped, heat-generating porous debris beds. This is achieved using a multiphase computational fluid dynamics (CFD) model that is implemented in the framework of the commercial CFD platform ANSYS FLUENT. The model is extensively validated with available experimental data on two different aspects: pressure drop in two-phase flow through porous media and prediction of dryout in typicalporous debris beds. A wide range of system pressure, relevant to severe accident conditions, is considered in this analysis in order to obtain a thorough understanding of its impact on multiphase flow and dryout occurrence inporous debris beds. This analysis is performed for different subcoolings of flooding water, which gives additional knowledge on the effects of coolant subcooling on dryout occurrence. Results indicate that dryout in debris beds occurs at progressively higher power densities as the system pressure is raised. Similar effects are observed with increase in liquid subcooling as well.

    • Flow reversal prediction of a single-phase square natural circulation loop using symbolic time series analysis


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      In the field of thermal engineering, one of the biggest concerns is the cooling of heat producing systems. For this purpose, today’s world is encouraging to use such cooling systems which are free from any active components (passive systems) for its high reliability and compact size. For this reason, to establishcooling by transferring heat from one place (source) to another (sink) passive system like natural circulation loop (NCL) is highly used. Fluid flow dynamics of the NCL is changing with the increase in heater power which is used as the source for the simulation. We found steady flow dynamics for the comparatively low power of heat, and with the rise in the power first, we saw the oscillatory flow dynamics and then found flow reversal characteristics. This paper presents a novel strategy for the early prediction of flow reversal phenomenon in NCLusing symbolic analysis of time series data. This time series data is found from the numerical simulation, and for the proper study, we are considering data after the initial transient part is overcome. Total time series data is transformed into a symbol string by partitioning into a finite number of specified symbolised groups. The state probability vector is calculated based on the number of occurrences of each symbol group. Present work is a single-phase study, and according to our geometry, we can provide a maximum 800 W heater power to stay in the single-phase. Therefore, for the early prediction of flow reversal in NCL, state probability vector evaluated at 800 W heater power which is the most undesirable state (chaotic data), and this is considered as the reference vector. The difference of the reference state vector from the current state vector is used as a parameter for early detection of flow reversal. It can be observed from the results that this difference changes significantly when the system is sufficiently away from the flow reversal.

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