A Correlation Model of Thermal Entrainment Factor for Single Air Curtain without Back Panel Airflow

This study presents a three-dimensional Computational Fluid Dynamics (CFD) simulation of the air flow pattern and the temperature distribution in a refrigerated display cabinet. The thermal entrainment is evaluated by the variations of the mass flow rate and thermal power along and across the air curtain considering the numerical predictions of abovementioned properties. The evaluation on the ambient air velocity for the three-dimensional (3D) effects in the pattern of this type of turbulent air flow is obtained. Additionally, it is verified that the longitudinal air flow oscillations and the length extremity effects have a considerable influence in the overall thermal performance of the equipment. The non uniform distribution of the air temperature and velocity throughout the re-circulated air curtain determine the temperature differences in the linear display space and inside the food products, affecting the refrigeration power of display cabinets. The numerical predictions have been validated by comparison with experimental tests performed in accordance with the climatic class n.° 3 of EN 441 Standard (Tamb = 25 °C, φamb = 60%; νamb = 0,2 m s−1). These tests were conducted using the point measuring technique for the air temperature, air relative humidity and air velocity throughout the air curtain, the display area of conservation of food products and nearby the inlets/outlets of the air mass flow.

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Dependency of Air Curtain Performance on Discharge Air Velocity (Grille and Back Panel) in Open Refrigerated Display Cabinets

Volume 9: Heat Transfer, Fluid Flows, and Thermal Systems, Parts A, B and C ◽

10.1115/imece2009-11029 ◽

2009 ◽

Cited By ~ 1

Author(s):

Pedro Dinis Gaspar ◽

L. C. Carrilho Gonc¸alves ◽

Andreas Vo¨geli

Keyword(s):

Heat Transfer ◽

Porous Medium ◽

Unit Length ◽

Experimental Tests ◽

Cfd Modeling ◽

Air Velocity ◽

Conservation Area ◽

Air Curtain ◽

Numerical Predictions ◽

Back Panel

This study performs a Computational Fluid Dynamics (CFD) modeling of air flow and heat transfer of an open refrigerated display cabinet in order to evaluate the influence of the discharge air velocity on the performance of its recirculated air curtain. The physical-mathematical model considers the flow through the internal ducts, across the fans and the evaporator, and also the thermal response of food products. The fan boundary condition is modeled in order to vary the air velocity at the discharge grille. The back panel perforation is modeled as a porous medium. The density and dimension of the back panel perforation variation is modeled by the Darcy’s law with the Forchheimer extension, varying the viscous and inertial resistance coefficients of the porous medium, based on its porosity, permeability, air velocity and pressure loss coefficient. Experimental tests were conducted to characterize the phenomena near the physical borders and to prescribe boundary conditions as well as to validate the numerical predictions on the temperature, relative humidity and velocity distributions. The numerical results show that the lowest average temperature in the conservation area of the display cabinet is achieved at a discharge air grille velocity of 1.15 ms−1. This value is lower than the experimental one, 1.51 ms−1, measured on the real equipment. The absence of a velocity component in the third dimension, which can destabilize the air curtain, is assumed to be the reason for this discrepancy. The profiles of the numerical predictions of the air curtain indicate that in the optimum case the air curtain is not so stable to bear big disturbances from outside. In order to increase the thermal performance and to reduce the energy consumption of these equipments, it’s not recommended to run the re-circulated air curtain velocity below 1.15 ms−1. For each CFD model, the values and directions of the air mass flow rate and heat transfer across the re-circulated air curtain by unit length are predicted and compared with the experimental ones in order to evaluate its thermal energy gains and losses.

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Experimental Analysis to Optimize the Performance of Air Curtains and Heat Exchangers

Handbook of Research on Advances and Applications in Refrigeration Systems and Technologies - Advances in Mechatronics and Mechanical Engineering ◽

10.4018/978-1-4666-8398-3.ch016 ◽

2015 ◽

pp. 590-640

Author(s):

Samuel Mariano do Nascimento ◽

Gustavo Galdi Heidinger ◽

Pedro Dinis Gaspar ◽

Pedro Dinho Silva

Keyword(s):

Mathematical Model ◽

Energy Consumption ◽

Heat Exchanger ◽

Thermal Performance ◽

Environmental Conditions ◽

Heat Exchangers ◽

Heat Load ◽

Experimental Studies ◽

Air Curtain ◽

Back Panel

This chapter reports an overview about experimental studies concerning the thermal performance of air curtains and heat exchangers installed in vertical open refrigerated display cases. The air curtain analysis shows the influence on the thermal performance by varying the width of the discharge air grille and the perforation density of the back panel by a mathematical model. The variation on the perforation density of the back panel and the width of discharge air grille alter significantly the thermal entrainment factor and the energy consumption of the equipment. Focusing the influence of environmental conditions on the performance of the heat exchanger, a second mathematical model was developed to evaluate the total heat load, its partial components and the condensate water mass. This analysis provides valuable information to the design of the air curtain and heat exchanger based on in-store environmental conditions and airflow efficiency.

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Entrainment by a Refrigerated Air Curtain Down a Wall

Journal of Fluids Engineering ◽

10.1115/1.1792263 ◽

2004 ◽

Vol 126 (5) ◽

pp. 871-879 ◽

Cited By ~ 7

Author(s):

Pratik Bhattacharjee ◽

Eric Loth

Keyword(s):

Reynolds Number ◽

Stokes Equations ◽

Reynolds Numbers ◽

Two Dimensions ◽

Transitional Flow ◽

Navier Stokes ◽

Wall Jets ◽

Adiabatic Wall ◽

Air Curtain ◽

Thermal Entrainment

Thermal entrainment is important as it adversely affects energy consumption and evaporator humidity levels of refrigerated air curtain display cases, often at transitional Reynolds numbers. In order to get a more fundamental understanding of the mean and unsteady thermal entrainment processes, the shelf structure of a display case has been idealized to that of a plane, adiabatic wall subjected to refrigerated wall jets at laminar and transitional flow conditions. The wall jets are studied at different inflow profiles, Reynolds numbers, and Richardson numbers to investigate the effect on thermal entrainment rates. The primary simulation technique was direct numerical simulation of the Navier–Stokes equations in two dimensions for the low and moderate Reynolds numbers (though three-dimensional simulations were also conducted). At higher Reynolds numbers, a conventional Reynolds averaged Navier–Stokes approach was employed, which was found to give reasonable agreement with the above approach at a wall jet (early-transitional) Reynolds number of 2000. In general, the results yielded a significant variation in entrainment as a function of Reynolds number, with a minimum occurring at flow speeds immediately prior to transition. The entrainment rates were also sensitive to the initial velocity distribution, whereby a constant gradient profile (where any local velocity-gradient peaks were minimized) provided the least entrainment. Entrainment was also found to decrease with increasing Richardson number.

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