Bio: P. Gupta is an academic researcher from National Institute of Technology, Durgapur. The author has contributed to research in topics: Subcooling & Boiling. The author has an hindex of 1, co-authored 1 publications receiving 19 citations.
TL;DR: In this paper, the authors analyze DWO in several boiling channels with varying lengths (Froude number) adopting moving node scheme and fixed node scheme (FNS) to evaluate the capability of the methods.
Abstract: The research on density wave oscillation (DWO) in boiling channels during the last few decades has been reviewed. Model reductions through lumped parameterization of the distributed channels have been exercised to compute nonlinear DWOs. In the present article, we attempt to analyze DWOs in several boiling channels with varying lengths (Froude number) adopting moving node scheme (MNS) and fixed node scheme (FNS). Relative performances of MNS and FNS have been analyzed to evaluate the capability of the methods. The analysis suggests that MNS is highly computationally efficient and has excellent convergence compared to FNS and finite difference method. Extended numerical oscillations have been observed in FNS. The analysis also suggests that DWOs are strongly coupled with the extent of inlet subcooling (boiling boundary), pressure drop and vapor quality. At high inlet subcooling, the ratio of time period to transit time is significantly higher than 2.0 (2.5–6.0) whereas at low inlet subcooling the ratio remains around 2.0. Numerical experiments on long boiling channels (low Froude number) and short ones (high Froude number) derives a clear difference that the short channels with high Froude number has “islands of instability” in Npch–Nsub plane and undergoes both supercritical and subcritical bifurcations, whereas the boiling channel with low Froude number undergoes only supercritical bifurcations. The effect of node numbers on marginal stability boundary (MSB) has been discussed. Increased speed of convergence is observed with higher number of nodes. With finer nodalizations, the region of instability extends. Extensive validations of the nonlinear models with reference experimental data and numerical results confirm that MNS satisfactorily predicts MSB, supercritical and subcritical bifurcations. Quasi-periodic en route to chaos has been detected in the boiling channel as a result of periodic perturbation of pressure drop (Eu). The same has been confirmed by the analysis of power spectrum density (PSD) and computation of Lyapunov exponents.
TL;DR: An updated review of two-phase flow instabilities including experimental and analytical results regarding density-wave and pressure-drop oscillations, as well as Ledinegg excursions, is presented in this article.
Abstract: An updated review of two-phase flow instabilities including experimental and analytical results regarding density-wave and pressure-drop oscillations, as well as Ledinegg excursions, is presented. The latest findings about the main mechanisms involved in the occurrence of these phenomena are introduced. This work complements previous reviews, putting all two-phase flow instabilities in the same context and updating the information including coherently the data accumulated in recent years. The review is concluded with a discussion of the current research state and recommendations for future works.
TL;DR: A systematic overview of all key two-phase instabilities focusing on the fundamental mechanisms leading to their occurrence is provided, with emphasis on how these mechanisms may change depending on whether flow may be classified as macro- or micro-channel.
Abstract: Study of two-phase flow instabilities began in the late 1920′s, and in the nearly 100 years since, significant progress has been made in both experimental and theoretical understanding of them. Despite these advances, many key deficiencies remain, solution of which will provide appreciable value for system designers looking to leverage phase change heat transfer technologies in a safe and repeatable manner. The present review provides a systematic overview of all key two-phase instabilities focusing on the fundamental mechanisms leading to their occurrence. Emphasis is placed on how these mechanisms may change depending on whether flow may be classified as macro- or micro-channel, particularly relevant due to the modern proliferation of parallel micro-channel heat sinks. Extensive literature surveys are performed for each instability type, and strengths and weaknesses of existing literature assessed. Focus is placed on providing recommendations for future work based on the status of current literature. Important takeaways include the significant mechanistic differences for Density Wave Oscillations and Parallel Channel Instability between macro- and micro-channels, the need for better understanding of the role of parallel micro-channels on external pressure curves (impacting Ledinegg instability and Pressure Drop Oscillations), and the influence of size and position of compressible volume on Pressure Drop Oscillations.
TL;DR: In this article, the authors investigated the flow and heat transfer instabilities in supercritical pressure n-decane, with pressure of 2.5 and 3.0 MPa and inlet temperature of 16-225°C.
Abstract: The fuel may be used in hypersonic vehicles to cool various surfaces; however, supercritical pressure fuels have been found to have deteriorated heat transfer rates and instabilities for some flow conditions. The flow and heat transfer instabilities in supercritical pressure n-decane were investigated experimentally in this work, with pressure of 2.5 and 3.0 MPa and inlet temperature of 16–225 °C. The heat flux was slowly increased to observe the flow in different stages as well as to obtain the boundary lines for a stability map. Seven stages were observed with different stability features. The transition to turbulence was found to be the main reason for the instability for stage b with slightly irregular oscillations, while dramatic variations of the thermal properties caused Helmholtz oscillations with regular frequencies and large amplitudes. The heat transfer deterioration in conjunction with an instability with buoyancy due to the density variation was found to be the reason. Higher pressures, inlet mass flow rates or fluid temperatures or downward flow weakened the instabilities, so these should be used in engineering designs to reduce the heat transfer deterioration and instability. The stability map provides further support for the nonlinear dynamic theory explaining the oscillations.
TL;DR: In this article, a study of non-linear stability analysis using co-dimension two (i.e., two free parameters being varied) bifurcations related to pressure drop oscillations in a heated channel has been carried out.
Abstract: A study of non-linear stability analysis using co-dimension two (i.e., two free parameters being varied) bifurcations related to pressure drop oscillations (PDO) in a heated channel has been carried out in this paper. In the existing literature, in the context of PDO, mostly linear stability analysis is done. A few works on non-linear stability analysis are available; however, only co-dimension one bifurcation studies of PDO have been carried out in these works. However, in practice, since the inlet temperature of the coolant is also an independent operating parameter, both inlet temperature, and inlet mass flow rate need to be considered for stability analysis. It is also noted that the existing plethora of studies on PDO is limited to the prediction of supercritical Hopf bifurcation only. However, in the current study, two types of Hopf bifurcations have been identified namely subcritical and supercritical. A subcritical Hopf bifurcation exhibits unstable limit cycles, which is a signature of the existence of unstable solutions for slightly larger perturbations even in the linearly stable region. The existence of unstable solutions indicates that linear stability analysis is not sufficient to identify the overall stability behavior. Also, a generalized Hopf point on the stability boundary has been identified which denotes a boundary between subcritical and supercritical Hopf bifurcation. Furthermore, numerical simulations are carried out in different regions (both stable and unstable) of the parameter space to understand the non-linear phenomena of the system. Moreover, the bifurcations are explained in terms of the interaction of the external and internal characteristic curves of the system. The large and small amplitude cycles are shown in the characteristic curves of the system.
TL;DR: In this paper, the authors evaluated the characteristics of the flow and heat transfer instabilities of n-decane at supercritical pressures under different inlet temperature, pressure, mass flow, and flow direction conditions.
Abstract: The characteristics of the flow and heat transfer instabilities of n-decane at supercritical pressures were evaluated under different inlet temperature, pressure, mass flow, and flow direction conditions. The experiment was carried out in a vertical heating tube with an inner diameter of 2 mm, at a maximum heat flux density of 200 kW/m2, under both laminar and turbulent inlet conditions. The results revealed that an increase in the inlet temperature, pressure, or mass flow rate can weaken the flow and heat transfer oscillation. Two oscillation stages of were observed, one is the transition oscillation that occurs when the fluid flows from laminar to turbulent, the other is the Helmholtz oscillation due to the uneven distribution of the radial density and the density distribution in the flow direction. Furthermore, a novel oscillation inhibition method was proposed, which involves the insertion of a twisted stainless-steel wire from certain parts of the heating tube. The influence of the insertion methods on the heat transfer enhancement and oscillation inhibition was evaluated using a comprehensive heat transfer coefficient. The results revealed that the insertion of a twisted wire from the tube inlet was more effective for the inhibition of the oscillation than that from the tube outlet. The insertion of a twisted wire into the entire length of the tube resulted in the best comprehensive performance of heat transfer and flow resistance when compared with those of other insertion methods.