Alain Kondjoyan

Effects of free stream turbulence intensity on heat and mass transfers at the surface of a circular cylinder and an elliptical cylinder, axis ratio 4

Abstract Effect of free stream turbulence intensity ranging from 1.5 to 40% and of air velocity ranging from 0.5 to 5.0 m s −1 on transfer coefficients has been measured for a circular cylinder ( d = 0.1 m) and an elliptical cylinder ( a = 0.2 m, R = 4) in cross-flow. The effect of turbulence intensity appeared to be as important as the influence of velocity and seemed to be independent of the pressure gradient and of the degree of turbulence isotropy. One relation for each of the two cylinders has been established to describe all the experimental data. It appeared also that the transfers are much less affected by the axis ratio of the cylinder than by the air flow properties.

Effect of cooking on protein oxidation in n-3 polyunsaturated fatty acids enriched beef. Implication on nutritional quality.

The effect of cooking on protein oxidation was investigated in M. Longissimus thoracis of eight Normand cows fed during a 100 days finishing period with two different diets: a conventional diet (concentrate/straw based diet) and a diet rich in n-3 polyunsaturated fatty acids (PUFAs), obtained by addition to the conventional diet of a mixture of extruded linseed and extruded rapeseed. After 11 days storage, at 4 degrees C under vacuum, meat was cooked by applying jets of steam. Three experimental heating treatments were tested: two with constant surface temperatures of 65 and 96 degrees C during 300s, and one with a continuous increasing surface temperature up to 207 degrees C. Protein oxidation was evaluated by the measurement of carbonyls, aromatic amino acids, and free thiols content. The formation of Schiff bases due to the reaction of proteins with aldehydic products of the lipid oxidation was also evaluated. Cooking resulted in a significant increase of carbonyl groups and Schiff bases as well as a significant degradation of tyrosine and tryptophan. Nevertheless, enrichment of the animal diet in n-3 PUFAs had minor effects on protein oxidation induced by cooking which are unlikely to be of nutritional significance.

Use of meat fluorescence emission as a marker of oxidation promoted by cooking.

Accumulation of fluorescent pigments in cooked bovine meat (M. Longissimus thoracis) was studied in relationship with the heating parameters (time and temperature). Muscles were aged at 4°C for 11days under vacuum before cooking. Meat cooking was performed by applying jets of steam. Three different heating treatments were tested: two with constant surface temperatures of 65 and 96°C for 300s, and one with a continuously increasing surface temperature up to 207°C. After extraction in water/dichloromethane/ethanol, fluorescence pigments were distributed between the apolar phase (emission 420-440nm after excitation at 360nm) and the polar phase, where two emission peaks were seen (emission 410-430 and 515nm after excitation at 360nm). Fluorescence in the two phases was little affected by heating at the two constant temperatures while it increased exponentially after 1min of treatment, as the varying temperature reached 141°C. The maximum fluorescence increases, measured in the extreme conditions of cooking (207°C/300s), were of 5000% in the apolar phase and 1700% in the polar phase. Thiobarbituric acid reactive substances (TBARS) and protein carbonyls were measured in parallel. The correlations between these two parameters and the fluorescence emission demonstrated that the interaction between proteins and aldehyde products of lipid peroxidation was mainly involved in the production of fluorescent pigments in cooked meat.

Heat and mass transfer coefficients at the surface of a pork hindquarter

Abstract Heat and mass transfer coefficients were measured at the surface of a pork hindquarter for common air flow properties encountered in chillers and dryers. Air velocity and turbulence intensity ranged from 0.4 to 5.0 m s −1 and 1.3% to 30%, respectively. The local values of the transfer coefficients can be verydifferent from one location to another at the body surface. An inversion of the air flow direction slightly affects the value of the mean transfer coefficients. The effect of turbulence intensity on the mean transfer coefficients is important, but complex and different from what happens on cylinders. An increase in turbulence intensity from 1.3% to 6% decreases the mean transfer coefficients values while a further increase in Tu increases those values. One relation has been established to describe the experimental data for Tu >- 6%.

Three-dimensional CFD calculations for designing large food chillers

In industry solid foods such as vegetables and meat are mostly air-chilled and stored in refrigerated rooms. Plant performance depends on heat and water exchanges between the air and the products and thus on the spatial distribution of the air parameters: velocity, turbulence intensity, temperature and relative humidity. In theory the transient behaviour of chillers could be calculated using computational fluid dynamics (CFD) codes, but current computer capacity and the inaccuracy of models for evaluating heat and mass transfer coefficients make them inapplicable to very large rooms containing hundreds of individual products. To overcome these limits a four-step calculation is proposed to assess how changes in chiller design and operating conditions affect chilling kinetics: (1) 3D calculation of the air velocity field with a CFD code, (2) deduction of an average air velocity map, (3) determination of heat and mass transfer coefficients in relation to the air velocity map using independent measurements, and (4) calculation of the product chilling kinetics using specific software. This procedure was applied to a pork chiller containing 290 carcasses. Two design cases differing in inlet air direction and flow rate, and two functioning modes, batch and continuous, were analysed. A fairly good agreement was observed between the calculated and measured air velocities. It was shown from temperature and weight loss kinetics that with this type of design, batch chilling can only be achieved overnight at the cost of overcooling for low weight carcasses, and increased weight losses.

Comparison of calculated and experimental heat transfer coefficients at the surface of circular cylinders placed in a turbulent cross-flow of air

Abstract Heat transfer coefficients have been calculated using computational fluid dynamics (C.F.D.) at the surface of an infinite circular cylinder (d = 0.1 m) subjected to a turbulent cross-flow of air whose velocity and turbulence intensities ranged from 0.5 to 5.0 m.s−1 and from 1.5 to 40%, respectively. The turbulence was accounted for by a k-ϵ model completed by near-wall treatment based either on a wall function or on Wolfshteins low-Reynolds number model. Results confirm that the wall function approach leads to great differences between calculated and experimental mean transfer coefficients. However, mean transfer coefficients are described efficiently by Wolfshteins model providing that the variation of the damping of turbulent viscosity with the boundary sub-layer thickness is considered. Finally this paper shows that the prediction of surface coefficients by C.F.D. calculations alone has to be taken with caution.

Determination of transfer coefficients by psychrometry

Abstract This paper presents a method based on psychrometry for measuring simultaneously heat and mass transfer coefficients in the case of forced convective exchanges between air and a body surface. This method is specially well Actapted to bodies of complex shapes. The theoretical aspects are described. Errors linked to data treatments and to measurements are discussed. The method is tested for a circular cylinder in cross flow in the following conditions: 3500 Re Tu = 12%, T air , = 25°C. Discussion shows that errors can be reduced to less than 2% for mean transfer coefficients and less than 5% for local coefficients.

Kinetic analysis of cooking losses from beef and other animal muscles heated in a water bath — Effect of sample dimensions and prior freezing and ageing

Cooking loss kinetics were measured on cubes and parallelepipeds of beef Semimembranosus muscle ranging from 1 cm × 1 cm × 1 cm to 7 cm × 7 cm × 28 cm in size. The samples were water bath-heated at three different temperatures, i.e. 50°C, 70°C and 90°C, and for five different times. Temperatures were simulated to help interpret the results. Pre-freezing the sample, difference in ageing time, and in muscle fiber orientation had little influence on cooking losses. At longer treatment times, the effects of sample size disappeared and cooking losses depended only on the temperature. A selection of the tests was repeated on four other beef muscles and on veal, horse and lamb Semimembranosus muscle. Kinetics followed similar curves in all cases but resulted in different final water contents. The shape of the kinetics curves suggests first-order kinetics.

Effect of high free stream turbulence on heat transfer between plates and air flows: A review of existing experimental results

Abstract This article reviews the research carried out to characterise the effect of free stream turbulence on heat transfer coefficients at the surface of a plate. In spite of the contradictions which appear at first glance, it seems that the transfer coefficient increases according to the free stream turbulence level either in the laminar boundary layer or in the turbulent boundary layer. The transfer coefficient increases regularly from turbulence level of 5% to 10% obtained with grid-type generators to the higher values of between 20% and 60% generated from jets. However describing the phenomenon using the sole effect of turbulence level leads to a large disparity of experimental results. Analysing the evolution of velocity and near wall temperature profiles under the influence of an increasing free stream turbulence leads to an interpretation which would make it possible to understand the nature of the phenomenon and explain the disparity of experimental results.

Combined heat transfer and kinetic models to predict cooking loss during heat treatment of beef meat

A heat transfer model was used to simulate the temperature in 3 dimensions inside the meat. This model was combined with a first-order kinetic models to predict cooking losses. Identification of the parameters of the kinetic models and first validations were performed in a water bath. Afterwards, the performance of the combined model was determined in a fan-assisted oven under different air/steam conditions. Accurate knowledge of the heat transfer coefficient values and consideration of the retraction of the meat pieces are needed for the prediction of meat temperature. This is important since the temperature at the center of the product is often used to determine the cooking time. The combined model was also able to predict cooking losses from meat pieces of different sizes and subjected to different air/steam conditions. It was found that under the studied conditions, most of the water loss comes from the juice expelled by protein denaturation and contraction and not from evaporation.