External Heat Transfer Coefficient in Agitated Vessels Using a Radial Impeller and Vertical Tube Baffles

2014 ◽  
Vol 53 (35) ◽  
pp. 13797-13803 ◽  
Author(s):  
Vitor da Silva Rosa ◽  
Marlene Silva de Moraes ◽  
Juliana Tófano de Campos Leite Toneli ◽  
Deovaldo de Moraes Júnior
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Vertical Tube ◽  
External Heat ◽  
External Heat Transfer

42 COMPARISON OF COOLING RATES IN OOCYTE VITRIFICATION SYSTEMS USING A NUMERICAL SIMULATION

2012 ◽  
Vol 24 (1) ◽  
pp. 133 ◽  
Author(s):  
M. Sansinena ◽  
M. V. Santos ◽  
N. Zaritzky ◽  
J. Chirife
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Cooling Rate ◽  
External Heat ◽  
Experimental Comparison ◽  
The Finite Element Method ◽  
Cooling Rates ◽  
External Heat Transfer

Interest in oocyte cryopreservation has increased due to the application of assisted reproductive technologies and the need for the establishment of ova/gene banks worldwide. In order to maintain cell viability, biological functions must be halted, inducing a suspended animation state by cooling it into a solid phase. Compared to cryopreservation of male gametes, oocytes represent a greater challenge due to their low surface area:volume. Vitrification, the solidification into an amorphous, glassy state while maintaining absence of intra- and extracellular ice crystals, requires high concentrations of cryoprotectants and extremely rapid cooling rates. Several vitrification devices such as open pulled straws (OPS), ultra fine pipette tips, nylon loops and polyethylene films have been introduced to manipulate minimal volumes and achieve high cooling rates. However, experimental comparison of cooling rates presents difficulties mainly because of the reduced size of these systems. To circumvent this limitation, a numerical simulation of cooling rates of various vitrification systems immersed in liquid nitrogen was conducted, solving the non-stationary heat transfer partial differential equation using the finite element method. Three external heat transfer coefficients (h = 200, 1000 and 2000 W m–2 K) were considered. The Cryotip® and OPS were approached as 2 concentric finite cylinders; differential equations representing heat transfer in cylindrical coordinates were described considering radial and axial coordinates and were numerically solved as a 1-dimensional heat conduction problem in an infinite cylinder. The Cryoloop® was approximated as a 1-dimensional heat flow system in Cartesian coordinates and Cryotop® was numerically described as an irregular bi-dimensional axial-symmetric problem. All differential equations were numerically solved using the finite element method in COMSOL Multiphysics 3.4. The domain was discretized in triangular (Cryotip®, OPS and Cryotop®) and linear elements (Cryoloop®) in order to obtain accurate numerical approximations. In each case, the warmest point of the system was identified to determine the time-temperature curve that allows the evaluation of the slowest cooling rate (worst condition). Results indicate the nylon loop (Cryoloop®) is the most efficient heat transfer system analysed, with a predicted cooling rate of 180 000°C min–1 for an external heat transfer coefficient h = 1000 W m–2 K when cooling from 20 to –130°C; in contrast, the pipette tips (Miniflex® showed the lowest performance with a cooling rate of 6164°C min–1 at same value of external heat transfer coefficient. Predicted cooling rates of OPS and Cryotop® (polyethylene film) were 40 909 and 37 500°C min–1, respectively for the same heat transfer coefficient. It can be concluded that in oocyte cryopreservation systems, in which experimental comparison of cooling rates presents difficulties due to the reduced size of the vitrification devices, the numerical simulations and the analysis of the predicted thermal histories could contribute to determine the performance of the different techniques.

Prediction of the Influence of Upstream Wake Passing on Downstream Blade Row External Heat Transfer Coefficient: A New Physically-Based Method

2010 ◽  
Author(s):  
K. S. Chana ◽  
B. R. Haller
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Gas Turbines ◽  
Cooling System ◽  
Guide Vane ◽  
External Heat ◽  
Test Facility ◽  
Internal Cooling ◽  
External Heat Transfer

For gas turbines, accurate prediction of the external heat transfer coefficient on the high pressure (HP) turbine rotor blades is of immense importance, as this component is critical and operates at material limits. Furthermore the external heat load is the governing boundary condition for the design of the internal cooling system of the blade. There is a continuous drive to increase the turbine entry temperature to increase the cycle efficiency, whilst developing blade cooling systems with higher efficiency (i.e. using less cooling air). A new systematic procedure has been developed and validated to predict the external heat transfer to a blade surface. The procedure allows for the unsteady effects caused by the passing of upstream nozzle guide vane (NGV) wakes. The early part of the suction surface was shown to have a pessimistic prediction of external heat transfer coefficient which resulted in unnecessary over-cooling of the blade in this region. The heat transfer aspect is found from the well-known TEXSTAN differential boundary layer method, developed by Mike Crawford at Texas University from the original approach of Spalding & Patankar. The method is validated against the MT1 turbine tested in the QinetiQ Turbine Test Facility. Predictions and comparisons have also been carried out on the VKI turbine stage. The level of agreement with the test data is shown to be good.

scholarly journals Turbine Airfoil External Heat Transfer Measurement in a Hot-Cascade

1997 ◽  
Author(s):  
Luzeng J. Zhang ◽  
Boris Glezer
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
External Heat ◽  
Operating Conditions ◽  
Test Facility ◽  
High Heat Flux ◽  
Transfer Coefficients ◽  
External Heat Transfer ◽  
Heat Transfer Coefficients

Detailed knowledge of local external heat transfer around a turbine airfoil is critical for an accurate prediction of metal temperature and component life. This paper discusses the results of measurements of the local gas side heat transfer coefficient distribution which were performed in an annular gas turbine vane hot-cascade test facility and provides comparisons with analytical predictions. The steady state tests were performed at simulated engine operating conditions. The tests were conducted on an internally cooled airfoil with the intent to provide close to uniform coolant temperature at the mid-span of the airfoil and also to obtain a high heat flux through the wall, minimizing uncertainty. Internal coolant side heal transfer coefficients were measured through calibration tests outside the cascade by a wire heater. A calibrated infrared pyrometer was used to provide detailed airfoil surface local temperatures and the corresponding external heat transfer coefficients. The airfoil was instrumented with a number of thermocouples for both calibration and temperature measurement. A 12% freestream turbulence was generated by a turbulence grid upstream of the test cascade. The static pressure measured over the test vane agreed well with invicid predictions. Heat transfer coefficients were measured and compared to a TEXSTAN boundary layer code prediction. The influence of Reynolds number and Mach number on the airfoil external heat transfer coefficient were also studied and is presented in the paper.

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