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) is unknown require a trial-and-error approach, starting with a guessed temperature to evaluate properties and , then refining the guess until convergence. Course Hero Key Equations Used
For flat horizontal surfaces, the characteristic length is calculated as surface area divided by perimeter:
Ra=Gr⋅Pr=gβ(Ts−T∞)Lc3ναcap R a equals cap G r center dot cap P r equals the fraction with numerator g beta open paren cap T sub s minus cap T sub infinity end-sub close paren cap L sub c cubed and denominator nu alpha end-fraction is the thermal diffusivity ( I can help walk you through the calculations
Plug the properties and characteristic length into the Rayleigh equation to assess the flow regime (laminar vs. turbulent). Step 6: Select the Appropriate Nusselt Number Correlation
Using the tables in Appendix A of Çengel's textbook (Table A-15 for air, Table A-9 for water), find the following properties at Tfcap T sub f Thermal conductivity ( Kinematic viscosity ( Prandtl number ( Volume expansion coefficient (
Evaluating heat loss from walls or vertical electronic circuit boards. turbulent)
A 10-cm-diameter, 20-cm-long cylinder is maintained at a temperature of 100°C in a large room where the temperature is 20°C. The heat transfer coefficient in free convection is to be determined.
varies non-linearly and must be looked up in thermodynamic property tables at the calculated film temperature. 2. Core Dimensionless Parameters In forced convection, the Reynolds number (
The correlation changes.
Understanding Heat and Mass Transfer: A Guide to Çengel’s 5th Edition Chapter 9 Introduction
) is known, solve for the convection heat transfer coefficient (
Do not just look at the final numerical result. Focus on the assumptions made, such as constant properties or steady-state conditions, and how properties were sourced from the appendix. The heat transfer coefficient in free convection is
Nu={0.60+0.387Ra1/6[1+(0.559/Pr)9/16]8/27}2cap N u equals the set 0.60 plus the fraction with numerator 0.387 cap R a raised to the 1 / 6 power and denominator open bracket 1 plus open paren 0.559 / cap P r close paren raised to the 9 / 16 power close bracket raised to the 8 / 27 power end-fraction end-set squared Horizontal Plates