Arnab Karmakar

Bio: Arnab Karmakar is an academic researcher from Birla Institute of Technology, Mesra. The author has contributed to research in topics: Subcooling & Boiling. The author has an hindex of 5, co-authored 6 publications receiving 44 citations. Previous affiliations of Arnab Karmakar include National Institute of Technology, Durgapur.

Lumped parameterization of boiling channel—Bifurcations during density wave oscillations

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.

Subcooled boiling oscillations in natural circulation boiling loop at low pressure

Abstract: Flow instabilities in a natural circulation boiling loop at a low pressure are reported. The oscillations at boiling incipience are primarily chaotic and bifurcate to quasiperiodic ones depending on inlet subcooling ΔTsub and heater power Q. They also strongly depend on water volume Φ in the loop. We have presented power spectrums, attractor reconstructions, and Hurst exponents for the analysis of the experimental data. The analysis shows that the primary oscillations are very similar to geysering instability. Chaotic oscillations occur at low ΔTsub or high Q, whereas quasiperiodic oscillations occur at high ΔTsub or low Q. Our experiments also suggest that wall superheat exceeding a critical value triggers the instability. © 2013 American Institute of Chemical Engineers AIChE J, 60: 375–386, 2014

Visualization of boiling flow structure in a natural circulation boiling loop

Abstract: The present study reports vapor–liquid jet flows, flow patterns and their transitions during geysering instability in a natural circulation boiling loop under varied inlet subcooling ΔTsub (30–50 °C) and heater power Q (4–5 kW). Video imaging, voltage measurement using impedance needle probe, measurement of local pressure and loop flow rate have been carried out in this study. Power spectra of the voltage, the pressure and the flow rate reveal that at a high ΔTsub the jet flows have long period (21.36–86.95 s) and they are very irregular with a number of harmonics. The period decreases and becomes regular with a decrease of ΔTsub. The periods of the jet flows at ΔTsub = 30–50 °C and Q = 4 kW are in close agreement with those obtained from the video imaging. The probe was found to be more efficient than the pressure sensor in detecting the jet flows within an uncertainty of 9.5% and in detecting a variety of bubble classes. Both the imaging and the probe consistently identify the bubbly flow/vapor-mushrooms transition or the bubbly flow/slug flow transition on decreasing ΔTsub or on increasing Q.

Nonlinear Analysis of Chaotic Time Series in a Natural Circulation Boiling Loop

Abstract: The present study reports chaotic flow oscillations observed in a natural circulation boiling loop. The periodic oscillations of wall temperature (thermal oscillations) initiate at a certain condition of heat flux and inlet subcooling in addition to geysering instability and pressure-drop oscillations. We observed that boiling regime changes from nucleate boiling to transition boiling as a result of decrease in inlet subcooling and increase in heat flux. The thermal oscillations are strongly coupled with pressure-drop oscillations. Nonlinear analysis of the time series of loop flow rate at various heater power and inlet subcooling have been carried out using statistical analysis, fast Fourier transform (FFT), time delay embedding for attractor reconstruction, autocorrelation, and correlation dimension. The analysis confirms that the oscillations are more chaotic at relatively low heater power and high inlet subcooling. The complexity of the oscillations strongly depends on boiling heat transfer regime. Our observations and analysis have been supported by other relevant experiments.

Effect of local phenomena on subcooled boiling oscillations in natural circulation boiling loop

Abstract: In this paper, the authors report the estimations of bubble frequency and oscillation of local void fraction and their role in subcooled boiling oscillations in a low-pressure natural circulation boiling loop The estimations primarily rely on the measurements of impedance using inductance L–capacitance C–resistance R (LCR) meter The bubble frequencies determined from the impedance signals and the images are comparable The effect of inlet subcooling on the bubble frequency and the oscillation of local void fraction has been studied and found to be remarkable Based on the comparison of the oscillations of local void fraction, local pressure and loop flow rate, the effect of local dynamic phenomena on the system oscillations clearly demonstrates that the oscillations of void fraction trigger high-amplitude flow oscillations with a delay between the oscillations of void fraction and loop flow rate

The growth of vapor bubbles in superheated liquids. report no. 26-6

Abstract: The growth of a vapor bubble in a superheated liquid is controlled by three factors: the inertia of the liquid, the surface tension, and the vapor pressure. As the bubble grows, evaporation takes place at the bubble boundary, and the temperature and vapor pressure in the bubble are thereby decreased. The heat inflow requirement of evaporation, however, depends on the rate of bubble growth, so that the dynamic problem is linked with a heat diffusion problem. Since the heat diffusion problem has been solved, a quantitative formulation of the dynamic problem can be given. A solution for the radius of the vapor bubble as a function of time is obtained which is valid for sufficiently large radius. This asymptotic solution covers the range of physical interest since the radius at which it becomes valid is near the lower limit of experimental observation. It shows the strong effect of heat diffusion on the rate of bubble growth. Comparison of the predicted radius‐time behavior is made with experimental observations in superheated water, and very good agreement is found.

Two-phase flow instabilities: A review

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.

Review of two-phase flow instabilities in macro- and micro-channel systems

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.

Experimental investigation of the flow and heat transfer instabilities in n-decane at supercritical pressures in a vertical tube

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.

Non-linear stability analysis of pressure drop oscillations in a heated channel

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.