Power LAB Report Mechanical Engineering

ENERGY AND POWER LABORATORY

LAB # 2 – Thermal Time Constant

Introduction

The objective of the lab is to calculate the time constant of the aluminum cylinder cooling in air, using the methods discussed in the lecture. Based on the time constant, the convective heat transfer coefficient (natural convection) is to be estimated using the lumped capacitance method.

Raw Data

The raw data for the heat-up and air cooling of the aluminum cylinder, provided in Excel Format, is available on TITANium.

The aluminum cylinder properties are as follows:

Diameter = 0.375 inch

Cp = 875 J/kg.K

= 2770bkg/m3

Cooled by natural convection in still air.

Simulated Results

Based on the simulated raw data provided, complete a memo report for the following:

· Report the time constant of the aluminum cylinder, cooling in air, using the “37% method” discussed in the lecture.

· Plot the temperature vs time data in terms of ln[(T-T∞)/T0-T∞)] versus seconds.

Here, T – time dependent cylinder temperature

T0 – initial temperature of cylinder

T∞ — measured air temperature.

Determine the time increment it takes the logarithmic temperature to drop one integer value. This will be approximately one time constant.

· Follow the steps outlined in the lecture to determine the time constant using a manual linear regression analysis. Confirm your results with a computer-based linear regression analysis.

· Calculate the free convective heat transfer coefficient of the cylinder cooling in air using the time constant obtained above from linear regression analysis. Calculate the Rayleigh number of the cylinder cooling the air. Use Equation (9.33) from your heat transfer textbook [1] to calculate a predicted heat transfer coefficient. How does the experimentally determined value differ from the empirical value? If the difference is significant, greater than 10 to 20 percent, why might the measured value be different?

Memo Report

Each student needs to submit an individual memo report for this laboratory.

The memo report is due by 4:00 PM on the due date specified in the Syllabus.

Include step-by-step sample calculations in the attachments.

Reference

- Bergman, T. L. Lavine, A. S. Incropera, Dewitt, D. P., 2011, Introduction to Heat Transfer, 6th Edition, John Wiley & Sons, New York.

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