Souvik Bhattacharyya

Steady state and dynamic simulation of multi-stage flash desalination plants: A case study

A mathematical model and its solution procedure are developed to simulate the steady-state and dynamic behaviour of multistage flash desalination plants. The steady-state and dynamic models are based on the same set of equations and are of the same order. The solution procedure developed is numerically stable and easy to implement. The steady state model is used to predict the operating parameters of a particular plant. A close agreement between the predicted values and the measured data from the actual plant has been observed. The transient response of the plant for step changes in input variables has been obtained. The computed response of the plant is comparable to the operating records of a real plant.

Feedback linearization based control of a variable air volume air conditioning system for cooling applications

Design of a nonlinear control system for a Variable Air Volume Air Conditioning (VAVAC) plant through feedback linearization is presented in this article. VAVAC systems attempt to reduce building energy consumption while maintaining the primary role of air conditioning. The temperature of the space is maintained at a constant level by establishing a balance between the cooling load generated in the space and the air supply delivered to meet the load. The dynamic model of a VAVAC plant is derived and formulated as a MIMO bilinear system. Feedback linearization is applied for decoupling and linearization of the nonlinear model. Simulation results for a laboratory scale plant are presented to demonstrate the potential of keeping comfort and maintaining energy optimal performance by this methodology. Results obtained with a conventional PI controller and a feedback linearizing controller are compared and the superiority of the proposed approach is clearly established.

Transcritical CO2 Heat Pump Dryer: Part 1. Mathematical Model and Simulation

In this study, a mathematical model and simulation code has been developed to investigate the performance of a transcritical CO2 heat pump dryer. The model takes into account detailed heat and mass transfer and pressure drop phenomena occurring in each component of the system. To take care of the variable heat transfer properties, the heat exchanger components were divided into several infinitesimal segments to examine the state, heat and mass balance and pressure drop for both refrigerant and air, and hence accurate results are expected. In Part 2 of the article, the model developed has been validated with experimental data and then the model was used to investigate effects of important operating parameters on the performance.

Corona Wind-Augmented Natural Convection--Part 1: Single Electrode Studies

Novel flow visualization and extensive and controlled heat transfer enhancement studies have been conducted to help understand and characterize the corona wind-augmented natural convection process from a vertical copper plate subject to a reversible polarity DC high-voltage electric field at various constant heating fluxes. The second part of the paper will address visualization studies such as particle entrainment, videography and Schlieren photography, and heat transfer enhancement employing multiple electrodes in a variety of configurations. Both positive and negative polarity DC high-voltage power supply at various heating fluxes was employed. The effects of varying a series of parameters coupled with a range of electrode voltages and its polarity have been investigated, and the trends associated with these experiments are discussed. Single electrode configurations were investigated to understand their impact on the enhancement phenomenon. Electric field strength had a direct and significant influence...

Rewetting Analysis of Hot Vertical Surfaces With Precursory Cooling by the Heat Balance Integral Method

The effect of precursory cooling on conduction-controlled rewetting of both slab and solid cylinder is analyzed by the heat balance integral method. A constant heat transfer coefficient is assumed in the wet region behind the wet front, while an exponentially decaying heat flux is assumed in the dry region ahead of the wet front. The physical problem is characterized by two dimensionless constants describing the extent of precursory cooling and three dimensionless numbers, namely, Peclet number, Biot number, and the nondimensional temperature. Results of the present solution are found to be in good agreement with other analytical solutions obtained through the Weiner-Hopf technique and the separation of variables as well as with the published experimental data for different coolants over a varied range of coolant flow rate. It is seen that precursory cooling increases the rewetting velocity particularly at higher flow rates. If it is neglected, the model grossly underpredicts the quench velocities.

Transcritical CO2 Heat Pump Dryer: Part 2. Validation and Simulation Results

The simulation model of a transcritical CO2 heat pump dryer presented in Part 1 has been first validated with available experimental data in this part and then used to simulate the heat pump dryer to study the variation of performance parameters such as heating COP, moisture extraction rate, and specific moisture extraction rate. The validation with experimental data shows that the model slightly over predicts the system performance. The possible reasons for the difference between experimental and numerical results are explained. Simulation results show the effect of key operating parameters such as bypass air ratio, re-circulation air ratio, dryer efficiency, ambient condition (temperature and relative humidity), and air mass flow rate. Results show that unlike bypass air ratio and ambient relative humidity, the effect of dryer efficiency, recirculation air ratio, ambient temperature, and air mass flow rate are very significant as far as the system performance is concerned.

Two-Phase Natural Circulation Loops: A Review of the Recent Advances

The wide range of applications of two-phase natural circulation loops (NCLs) demands intense analyses of boiling systems, and consequently a large number of experimental and theoretical research findings are available in the literature. This comprehensive review focuses on the state-of-the-art appraisal of the related technology, with a primary focus on studies carried out in the last two decades. General modeling strategies have been reported and compared. Different forms of thermal-hydraulic instabilities appearing in two-phase NCLs have been summarized, with discussions on possible ways of identification and their differences. In addition, studies on system instability are presented in a categorywise grouping following a chronological style in terms development of knowledge. Coupled nuclear instabilities are reported as well in some detail. Finally, this review identifies the gray areas where the information is not quite complete and suggests the required path of future research.

A universal optimum work rate potential for continuous endoreversible Carnot heat engine cycles

The theory of finite‐time thermodynamics for continuous endoreversible cycles gives rise to an optimum efficiency at maximum power output of η=1−(TL/TH)0.5 in contrast to the upper limit of η=1−(TL/TH) obtained from infinite‐time thermodynamics. It is shown here that, additionally, for continuous Carnot cycles, the finite‐time optimum rate of work output (Ẇopt), is exactly half of that obtained for infinite‐time reversible cycles (Carnot work rate, Ẇrev) operating between the same temperature limits (i.e. Ẇopt=1/2Ẇrev). This expression is true for both unoptimized as well as optimized mass flow rate conditions. The formulation used in the analysis is universal for all working substances for cycles using linear heat transfer laws.

Dynamic Performance Analysis of a Compressor Driven Metal Hydride Cooling System

Though several studies have been reported to show that compressor driven metal hydride cooling systems are competitive with conventional vapor compression systems, an elaborate computational model that takes into account the transient nature of the compressor and the conditioned space is still unreported. The results presented here are obtained for a room air conditioner with Zr0.9Ti0.1Cr0.55Fe1.45 as the hydrogen absorbing material and employing standard heat exchanger configurations. Though previous thermodynamic and transient studies predicted attainment of significant coefficients of performance, the present results indicate that even with the optimal design the maximum coefficient of performance and specific power will be around 2.38 and 750kJ∕kg-alloy.h, respectively. This indicates a need for better materials and effective control strategies so that these systems can become commercially viable.

Investigation of forced convective heat transfer enhancement in the presence of an electric field – a finite element analysis

The effect of electro‐convection in a cylindrical flow annulus on heat transfer enhancement has been investigated numerically. Weakly ionized air is considered to be the working fluid throughout this work. The effect of exit boundary condition, considered to be the main hindrance for the numerical solution, has been discussed in detail. The present work shows interesting flow field characteristics, which are in excellent agreement with some other established experiments. The heat transfer enhancement, as reported in this work, appears to be small in view of the low magnitude of the applied voltage but it clearly and surely delineates the trend, i.e. with increase in the strength of the electric field, heat transfer enhances.