Subhashis Ray

Friction and heat transfer characteristics of flow through square duct with twisted tape insert

This paper presents numerical prediction of characteristics of laminar, as well as, turbulent flow and heat transfer in a square sectioned duct inserted with a twisted tape, whose width equals the length of the duct side. The heat transfer characteristics are predicted under axially and peripherally constant wall heat flux conditions. As such, the flow and heat transfer are periodically fully developed in axial direction. Correlations for friction factor and Nusselt number are derived from the predicted data. The correlation for friction is compared with the experimental data, which are found to be in reasonably good agreement with each other, for both laminar and turbulent flows.

Laminar flow and heat transfer through square duct with twisted tape insert

Abstract Numerically predicted characteristics of laminar flow and heat transfer through square duct with twisted tape insert are presented in this paper. The transport equations are solved on a non-staggered non-orthogonal grid using the curvilinear version of the “Complete Pressure Correction” algorithm. Both the flow and heat transfer are in a state of periodic development in the axial direction. The heat transfer characteristics are predicted under axially and peripherally constant wall heat flux conditions. Correlations for friction factor and Nusselt number are developed from the predicted data. The agreement between the correlation and experimental data for friction factor is found to be excellent. Relative thermo-hydraulic performance of square and circular ducts, both fitted with twisted tapes of same twist ratio is also presented in this paper. The study shows significant improvements can be achieved with the square duct, particularly at higher Prandtl numbers and lower twist ratios. Improvement is observed even for air, over certain ranges of Reynolds number.

Mass flow-rate control through time periodic electro-osmotic flows in circular microchannels

The present study is directed towards devising a scientific strategy for obtaining controlled time-periodic mass flow-rate characteristics through the employment of pulsating electric fields in circular microchannels by exploiting certain intrinsic characteristics of periodic electro-osmosis phenomenon. Within the assumption of thin electrical double layers, the governing equations for potential distribution and fluid flow are derived, corresponding to a steady base state and a time-varying perturbed state, by assuming periodic forms of the imposed electrical fields and the resultant velocity fields. For sinusoidal pulsations of the electric field superimposed over its mean, a signature map depicting the amplitudes of the mass flow rate and the electrical field as well as their phase differences is obtained from the theoretical analysis as a function of a nondimensional frequency parameter for different ratios of the characteristic electric double layer thickness relative to the microchannel radius. Distinctive characteristics in the signature profiles are obtained for lower and higher frequencies, primarily attributed to the finite time scale for momentum propagation away from the walls. The signature characteristics, obtained from the solution of the prescribed sinusoidal electric field, are subsequently used to solve the “inverse” problem, where the mass flow rate is prescribed in the form of sinusoidal pulsations and the desired electric fields that would produce the required mass flow-rate variations are obtained. The analysis is subsequently extended for controlled triangular and trapezoidal pulsations in the mass flow rate and the required electric fields are successfully obtained. It is observed that the higher the double layer thickness is in comparison to the channel radius, the more prominent is the deviation of the shape of the required electric field pulsation from the desired transience in the mass flow-rate characteristics. Possible extensions of the analysis to more complicated pulsation profiles are also outlined.

Role of streaming potential on pulsating mass flow rate control in combined electroosmotic and pressure-driven microfluidic devices

In the present study, we investigate the implications of streaming potential on the mass flow rate control in a microfluidic device actuated by the combined application of a pulsating pressure gradient and a pulsating, externally applied, electric field. We demonstrate that the temporal dynamics due to streaming potential effects may lead to interesting non‐trivial aspects of the resultant transport characteristics. Our results highlight the importance of an adequate accounting of the streaming potential effects for temporally tunable mass flow rate control strategies, which may act as a useful design artifice to augment mass flow rates in practical scenarios.

Semianalytical solutions of laminar fully developed pulsating flows through ducts of arbitrary cross sections

A semianalytical analysis of fully developed pulsating flows in pipes of noncircular cross section is presented. The flow is assumed to be pressure gradient driven. Details of the analytical treatment of the flow are presented and it is shown that the analysis can be employed for any arbitrary cross-sectional shape. Special considerations are given to laminar pipe flows with circular cross sections and results of the present analysis are compared with those of conventional analytical treatments of the flow. Comparison of the velocity distribution, mass flow-rate pulsation, ratio of amplitudes of the mass flow rate and the pressure gradient, and the corresponding phase lag obtained by the present treatment show excellent agreement with data in the literature. Similar comparisons are also performed for oscillating flow through ducts of square and rectangular cross sections. Numerical data are introduced to confirm the resultant analysis. Finally, a few test cases are presented for sinusoidally pulsating flo...

Method for defined mass flow variations in time and its application to test a mass flow rate meter for pulsating flows

In a previous publication, the authors presented a transient mass flow rate metering technique for pulsating pipe flows and its outstanding performance was demonstrated experimentally for fuel injection nozzles. In the present paper, the application of the mass flow rate metering technique is described for pulsating air flows. The basics of the measuring technique are summarized and the corresponding experimental setup is explained. For verification experiments, a mass flow rate control unit was developed that permits time-varying mass flow rates to be provided proportional to the voltage of an electronic input signal to the unit. The basic ideas of this unit and its performance are summarized. Its application to verify the performance of the developed mass flow rate measuring instrument represents the major part of the paper. Performance tests were carried out for various time variations of mass flow rate pulsations. It is shown that the mass flow rate of the mean flow and that of the pulsating flow can be separated and both can be accurately measured. Comparative measurements show that the mass flow rate measuring technique works very well and reproduces the mass flow rate variations in time imposed by the mass flow rate control unit.

ANALYSIS OF LAMINAR NATURAL CONVECTION FROM A DISCRETE ISOTHERMAL FLUSH HEATER MOUNTED ON THE SIDE WALL OF A PARTIALLY OPEN RECTANGULAR ENCLOSURE

A numerical study is performed for natural confection cooling of a discrete isothermal heater located on one vertical wall of a partially open two-dimensional enclosure. The governing equations are solved using the SIMPLER algorithm. Studies are performed for 103 <Ra <106 with air as the cooling medium for different heater locations and barrier heights. The primary circulation is stronger for higher Ra and lower heater positions. The barrier strongly affects the size and the strength of the recirculation formed near the barrier, although its effect on the bulk circulation and heat transfer is negligible. Correlations of average Nusselt number are developed for different heater locations and barrier heights.

Simulation of Complete Liquid-Vapor Phase Change inside Divergent Porous Evaporator

Complete phase change process within a divergent porous evaporator is numerically investigated in this paper. A smoothing algorithm, proposed by the present authors, is used in order to deal with the discontinuity in effective diffusion coefficient. Effects of various parameters on the temperature distribution are carefully investigated, which clearly indicate that operating conditions and the geometry of diffuser strongly influence the outlet condition of steam, whereas, porous media properties have only minor impact.

Proposed method for measurement of flow rate in turbulent periodic pipe flow

The present investigation deals with a previously proposed flow metering technique for laminar, fully-developed, time-periodic pipe flow. Employing knowledge of the pulsation frequency-dependent relationship between the mass flow rate and the pressure gradient, the method allows reconstruction of the instantaneous mass flow rate on the basis of a recorded pressure gradient time series. In order to explore if the procedure can be extended for turbulent flows, numerical simulations for turbulent, fully-developed, sinusoidally pulsating pipe flow with low pulse amplitude have been carried out using a ?2-f turbulence model. The study covers pulsation frequencies, ranging from the quasi-steady up to the inertia-dominated frequency regime, and three cycle-averaged Reynolds numbers of 4360, 9750 and 15400. After providing the theoretical background of the flow rate reconstruction principle, the numerical model and an experimental facility for the verification of simulations are explained. The obtained results, presented in time and frequency domain, show good agreement with each other and indicate a frequency dependence, similar to that used for the signal reconstruction for laminar flows. A modified dimensionless frequency definition has been introduced, which allows a generalised representation of the results considering the influence of Reynolds number.

Response of Laminar Premixed Flame to Mass Flow Rate and Pressure Fluctuations

In this work, we develop an analytical model to describe laminar premixed flame response to an oscillating flow and use this model to predict the relationship between the heat release rate and the instantaneous flow field. Fully developed pulsating flow through a channel is considered. The flow is driven by pressure gradients. To facilitate direct comparison with experiments, the transient velocity profile is obtained in terms of mass flow rate fluctuations. The flame is anchored at the channel wall. The flame is assumed to be a thin surface, separating the reactants and the products. Flame displacement speed is assumed to be constant. The flame displacement is described by a single-valued function of the transverse coordinate. The flame dynamics is represented by a kinematic equation describing the displacement of the surface. The assumption of constant flame speed and fully developed flow allows closed-form solution of the flame response. The temporal variation of the mass flow rate and the flame surface area are compared to determine the gain and phase difference of the flame transfer function, relating the fluctuations in flame surface area to fluctuations in the mass flow rate.Copyright