Sujoy Kumar Saha

HEAT-TRANSFER AND PRESSURE-DROP CHARACTERISTICS OF TURBULENT-FLOW IN A CIRCULAR TUBE FITTED WITH REGULARLY SPACED TWISTED-TAPE ELEMENTS

Abstract Laminar flow heat transfer and pressure drop characteristics in a circular tube fitted with regularly spaced twisted-tape elements connected by thin circular rods were investigated experimentally. These characteristics are governed by Reynolds number, Prandtl number, twist ratio, space ratio, and rodto-tube diameter ratio. Correlations for friction factor and Nusselt number are also reported. It is shown that on the basis of both constant pumping power and constant heat duty, regularly spaced twisted-tape elements perform significantly better than full-length twisted tapes at high Reynolds numbers, high twists, and small spacings.

Thermohydraulic Study of Laminar Swirl Flow Through a Circular Tube Fitted With Twisted Tapes

Heat transfer and pressure drop characteristics in a circular tube fitted with twisted tapes have been investigated experimentally. Laminar swirl flow of a large Prandtl number (205<Pr<518) viscous fluid was considered. The swirl was generated by short-length twisted-tape inserts; regularly spaced twisted-tape elements with multiple twists in the tape module and connected by thin circular rods; and smoothly varying (gradually decreasing) pitch twisted-tapes. The heat transfer test section was heated electrically imposing axially and circumferentially constant wall heat flux (UHF) boundary condition Reynolds number, Prandtl number, twist ratio, space ratio, number of tuns in the tape module, length of the twisted-tape and smoothness of the swirling pitch govern the characteristics. Friction factor and Nusselt number are lower for short-length twisted-tape than those for full-length twisted-tape

Turbulent flow and heat transfer from a slot jet impinging on a moving plate

Abstract The flow field due to an impinging jet over a moving surface at a moderately high Reynolds number, emanating from a rectangular slot nozzle has been computed using the large eddy simulation technique. A dynamic subgrid-scale stress model has been used for the small scales of turbulence. The velocity of the impinging surface perpendicular to the jet velocity has been varied up to two times the jet velocity at the nozzle exit. Turbulence quantities such as kinetic energy, production rate of turbulent kinetic energy and the Reynolds stresses are calculated for different surface velocities. It has been observed that, while the turbulent kinetic energy increases with increasing velocity of the impinging surface, production rate of turbulence initially increases with increasing surface velocity and then comes down. By analyzing the components of turbulent production it was found that P 33 is the dominant term up to the surface velocity of one unit and when the surface velocity is two times the jet velocity at the nozzle exit, the major contribution to turbulence production comes from P 13 and partly from P 11 . Heat transfer from the plate initially increases with non-dimensional surface velocity up to 1.2 and then comes down.

Numerical prediction of laminar flow and heat transfer characteristics in a tube fitted with regularly spaced twisted-tape elements

Abstract By numerically solving the Navier-Stokes and energy equations in their three-dimensional parabolic form, the friction and heat transfer characteristics are predicted for laminar flow in a circular tube fitted with regularly spaced twisted-tape elements that are connected by circular rods. The predictions have agreed closely with the experimental data for water. It is shown that a strong L D effect is associated with the configuration under consideration. By extending the predictions outside the range of experimental data, it is found that considerably enhanced thermohydraulic performance is achievable by increasing the number of turns on the tape elements, by reducing the connecting rod diameter, and at high fluid Prandtl numbers.

Thermohydraulics of Laminar Flow Through Rectangular and Square Ducts With Transverse Ribs and Twisted Tapes

The heat transfer and the pressure drop characteristics of laminar flow of viscous oil through rectangular and square ducts with internal transverse rib turbulators on two opposite surfaces of the ducts and fitted with twisted tapes have been studied experimentally. The tapes have been full length, short length, and regularly spaced types. The transverse ribs in combination with full-length twisted tapes have been found to perform better than either ribs or twisted tapes acting alone. The heat transfer and the pressure drop measurements have been taken in separate test sections. Heat transfer tests were carried out in electrically heated stainless steel ducts incorporating uniform wall heat flux boundary conditions. Pressure drop tests were carried out in acrylic ducts. The flow was periodically fully developed in the regularly spaced twisted-tape elements case and decaying swirl flow in the short-length twisted tapes case. The flow characteristics are governed by twist ratio, space ratio, and length of twisted tape, Reynolds number, Prandtl number, rod-to-tube diameter ratio, duct aspect ratio, rib height, and rib spacing. Correlations developed for friction factor and Nusselt number have predicted the experimental data satisfactorily. The performance of the geometry under investigation has been evaluated. It has been found that on the basis of both constant pumping power and constant heat duty, the regularly spaced twisted-tape elements in specific cases perform marginally better than their full-length counterparts. However, the short-length twisted-tape performance is worse than the full-length twisted tapes. Therefore, full-length twisted tapes and regularly spaced twisted-tape elements in combination with transverse ribs are recommended for laminar flows. However, the short-length twisted tapes are not recommended.

Heat Transfer and Pressure Drop Characteristics of Laminar Flow Through a Circular Tube With Longitudinal Strip Inserts Under Uniform Wall Heat Flux

Heat transfer and pressure drop characteristics in a circular tube fitted with full-length strip, short-length strip, and regularly spaced strip elements connected by thin circular rods have been investigated experimentally. The strips have been rectangular, square and crossed in cross-section with different aspect ratio. Laminar flow of water and other viscous liquids was considered. The rod diameter and length of the strip-rod assembly and the length of the strips were varied. Isothermal friction factor data has been generated. The heat transfer test section was heated electrically imposing axially and circumferen-tially constant wall heat flux (UWHF) boundary condition. Reynolds number, Prandtl number, strip length, strip ratio, space ratio, and rod-diameter govern the characteristics. Smaller rod-diameter in the strip-rod assembly or pinching of the strips in place rather than connecting the strip elements by rods performs better thermohydraulically. Short-length strips (upto a limited fraction of the test section tube length) perform better than the full-length strip. The friction factor correlation and the correlation for Nusselt number under UWHF condition for full-length strip have been modified to make them suitable for short-length strip as well as regularly-spaced strip elements. Thermal entrance length in the correlations is represented by Graetz number. Friction factor and Nusselt number correlations for short-length strips as well as regularly-spaced strip elements, in the limit. reduce to their full-length counterparts.

Simulation of laminar slot jets impinging on a moving surface

Laminar flow and heat transfer on a moving surface due to a bank of impinging slot jets have been numerically investigated. Two types of jet, namely axial and knife-jet with an exit angle of 60 deg were considered. The surface velocity up to two times the jet velocity at the nozzle exit was imposed on the impinging surface. It has been observed that while with increasing velocity of the impinging surface, the total heat transfer reduces; the distribution pattern becomes more uniform. For the same amount of mass and momentum flux at the nozzle exit, heat transfer from the axial jet is considerably higher than that from the vectored jets at all surface velocities considered

Thermohydraulics of Laminar Flow Through Rectangular and Square Ducts With Axial Corrugation Roughness and Twisted Tapes With Oblique Teeth

The heat transfer and the pressure drop characteristics of laminar flow of viscous oil (175 <Pr <538) through rectangular and square ducts with combined internal axial corrugations on all the surfaces of the ducts and with twisted-tape inserts with and without oblique teeth have been studied experimentally. The axial corrugations in combination with both twisted tapes with and without oblique teeth have been found to perform better than either axial corrugations or twisted-tape inserts acting alone. The heat transfer and the pressure drop measurements have been taken in separate test sections. Heat transfer tests were carried out in electrically heated stainless steel ducts incorporating uniform wall heat flux boundary conditions. Pressure drop tests were carried out in acrylic ducts. The flow friction and thermal characteristics are governed by duct aspect ratio, corrugation angle, corrugation pitch, twist ratio, space ratio, length, tooth horizontal length and tooth angle of the twisted tapes, Reynolds number, and Prandtl number. Correlations developed for friction factor and Nusselt number have predicted the experimental data satisfactorily. The performance of the geometry under investigation has been evaluated. It has been found that based on constant pumping power, up to 45% heat duty increase occurs for the combined axial corrugation and twisted-tape insert case compared with the individual axial corrugation and twisted-tape insert cases in the measured experimental parameters space. On the constant heat duty basis, the pumping power has been reduced up to 30% for the combined enhancement geometry than the individual enhancement geometries.

NUMERICAL INVESTIGATIONS OF HEAT TRANSFER OVER A MOVING SURFACE DUE TO IMPINGING KNIFE-JETS

Turbulent flow field and heat transfer from an array of impinging horizontal knife jets on a moving surface have been investigated using large eddy simulation (LES) with a dynamic subgrid stress model. The surface velocity directed perpendicular to the jet plane is varied up to two times the jet velocity at the nozzle exit. Performance of a horizontal knife jet with an exit angle of 60° is compared with the standard axial jet. It has been observed that increasing surface motion reduces heat transfer for both types of jets. However, the amount of heat transfer from the knife jets is more than that from the axial jets when the surface velocity is within the order of half the jet velocity at the nozzle exit. For further increase in surface velocity, heat transfer from the knife jets is, however, less than that in the case of axial jets if the Reynolds number (Re) is low. For higher Re and higher surface velocity, the heat transfer from either type of jets is of comparable magnitude.Turbulent flow field and heat transfer from an array of impinging horizontal knife jets on a moving surface have been investigated using large eddy simulation (LES) with a dynamic subgrid stress model. The surface velocity directed perpendicular to the jet plane is varied up to two times the jet velocity at the nozzle exit. Performance of a horizontal knife jet with an exit angle of 60° is compared with the standard axial jet. It has been observed that increasing surface motion reduces heat transfer for both types of jets. However, the amount of heat transfer from the knife jets is more than that from the axial jets when the surface velocity is within the order of half the jet velocity at the nozzle exit. For further increase in surface velocity, heat transfer from the knife jets is, however, less than that in the case of axial jets if the Reynolds number (Re) is low. For higher Re and higher surface velocity, the heat transfer from either type of jets is of comparable magnitude.

THERMOHYDRAULICS OF LAMINAR FLOW THROUGH A CIRCULAR TUBE HAVING INTEGRAL HELICAL CORRUGATIONS AND FITTED WITH HELICAL SCREW-TAPE INSERT

The experimental friction factor and Nusselt number data for laminar flow through a circular duct having integral helical corrugations and fitted with a helical screw-tape insert are presented. Predictive friction factor and Nusselt number correlations are also presented. The thermohydraulic performance was evaluated. The major findings of this experimental investigation are that the helical screw-tape insert in combination with integral helical corrugations performs significantly better than the individual enhancement technique acting alone for laminar flow through a circular duct up to a certain value of the fin parameter. This research finding is useful in designing tubes carrying solar thermal mass of viscous oil in a parabolic trough solar collector used in environmentally sound and increasingly cost-effective solar thermal electric power plants. The result is also useful in designing heat exchangers used in process industries.