Drying heat-transfer coefficient scaling with air mass velocity: for constant-rate drying over a surface, hG = 0.0176 * G^0.8 for parallel flow. What correlation applies when air impinges perpendicular to the surface?

Introduction / Context:During the constant-rate period of drying, convective heat transfer from air to a wet surface dominates. Empirical correlations relate the heat-transfer coefficient per unit mass flux (hG) to air mass velocity G. Flow orientation matters: parallel flow over a plane differs from perpendicular impingement jets hitting the surface.

Given Data / Assumptions:

• Constant-rate drying region; surface is saturated with liquid water or solvent.
• Air properties are evaluated at film conditions; negligible radiation.
• Correlations are empirical and dimensionally consistent for hG in Kcal/hr·m^2·°C and G in kg/hr·m^2 (units as stated in the stem).

Concept / Approach:Parallel flow correlations often show a stronger dependence on G (exponent near 0.8), while impingement jets provide high local turbulence but the scaling exponent tends to be lower, reflecting different boundary-layer development and stagnation-point heat transfer behavior for the metric hG defined here. A widely cited switch is from 0.0176 * G^0.8 (parallel) to 1.004 * G^0.37 (perpendicular).

Step-by-Step Solution:Identify the given base: hG_parallel = 0.0176 * G^0.8.Apply orientation change to perpendicular impingement: hG_perp = 1.004 * G^0.37.Confirm units and exponents align with empirical data sets for impinging flow conditions.

Verification / Alternative check:Drying literature and equipment vendor data show reduced exponent values for impingement configurations when hG is expressed per unit G in the given units, matching the presented correlation.

Why Other Options Are Wrong:(b) keeps the 0.8 exponent, which suits parallel flow; (c) uses the small coefficient with the impingement exponent; (d) is unrealistic; (e) contradicts extensive empirical evidence.

Common Pitfalls:Mixing SI and English units; applying correlations outside their validity ranges (very low or high G); ignoring humidity effects on properties.

Final Answer:hG = 1.004 * G^0.37