Non-uniform Heat Transfer During Air-Blast Freezing of a Fruit Pulp Model in Multilayer Boxes

• Published in: Food and bioprocess technology 2013, Vol.6 (1), p.146-159
• Main Authors: de Resende, Jaime V., Prado, Mônica E. T., Junior, Vivaldo Silveira
• Format: Article
• Language: English
• Subjects: Accuracy; Aerial explosions; Agriculture; Air flow; Air temperature; Aluminum; Biotechnology; Boundary layers; Boxes; Chemistry; Chemistry and Materials Science; Chemistry/Food Science; Computer applications; Convective heat transfer; Diameters; Ducts; Food preservation; Food Science; Freezing; Fruits; Heat transfer; Heat transfer coefficients; Mathematical models; Multilayers; Original Paper; Pulp; Temperature profiles
• ISSN: 1935-5130; 1935-5149
• DOI: 10.1007/s11947-011-0757-6

Effective heat transfer coefficients were measured using an aluminum test body and compared with the results obtained from a Gnielinski correlation for air-blast freezing of a fruit pulp model in multilayer boxes, with the internal airflow through rectangular ducts and the hydraulic diameter as characteristic dimensions. The quantities of products inside the boxes were varied, and the inlet air velocities and temperature profiles during freezing were measured. The inlet air velocities were applied in dimensionless Gnielinski correlations to estimate the local heat transfer coefficient values. The experimental and predicted heat transfer coefficient values were used to determine an average convective heat transfer coefficient weighted by the heat transfer area. The results from this methodology were used in an analytically derived procedure for freezing-time estimates and then compared with experimental results. The average effective heat transfer coefficient underestimated freezing times and demonstrated a higher level of accuracy than the Gnielinski correlation when applied to boxes containing smaller product amounts. For experiments with greater quantity of products, the use of average heat transfer coefficients from the Gnielinski correlation yielded errors lower than 20%. Based on boundary layer theory, the Gnielinski correlation can be used to explain the isotherm behaviors observed during freezing. Many of the results satisfy the standards of accuracy used in engineering, and the procedure does not require extra computational effort.