Measurement of Thermal Conductivity of Liquids and Gases

Measurement of Thermal Conductivity of Liquids and Gases Ajith Krishnan.R Roll No-2 GEC Kozhikode

Content Layout • Why Thermal Conductivity Measurement important? • General Principles in Thermal Conductivity Measurement o Steady state Methods o Unsteady state Methods

• Guarded Hot plate Apparatus • Radial Heat Conduction Apparatus • Transient Hot Wire Method

Why Thermal Conductivity Measurement important • Thermal conductivity(k) - Transport property • Essential for Energy Balance calculations in Heat transfer applications. • Thermal Conductivity defined by Fourier Equation

𝒒𝒙 = −𝒌 𝑨

𝝏𝑻 𝝏𝒙

,

k (W/m.K)

General Principles in Thermal Conductivity Measurement • • • •

One dimensional temperature field assumption Measure Heat Flux Measure Temperature Gradient In liquids and gases Supress Convection

Steady state Methods ( more time , expensive controller) o Guarded Hot Plate Method o Radial Heat Flow Apparatus

Unsteady state Methods (less time, expensive) o Transient Hot Wire Method

Figure1:Simple thermal-conductivity measurement (Experimental Methods for Engineers, J. P. Holman)

Guarded Hot plate Apparatus • Immobilizing the fluid o Use a thin layer of the fluid o Temperature field - hot part is above the cold part

• The guarded hot plate apparatus to achieve these two conditions. • Liquid film-0.05cm • Mostly used for liquids 𝑄 𝑘=

∆𝑇

𝐴 𝛿

Figure2:Guarded hot plate apparatus (http//nptel.iitm.ac.in/courses/IITMADRAS/Mechanical_Measurements_Metrology)

Radial Heat Conduction Apparatus • Radial flow of heat through a very thin layer of liquid or gas o o o o o

Diameter of cartridge heater D = 37 mm Radial clearance = 0.3 mm Heat flow area A = 0.0133 𝑚2 Temperature difference across the gap ΔT ~ 5°C Heater input Q = 20 – 30 W

k=

𝑟 𝑞.ln( 2 𝑟1 ) 2𝜋𝐿∆𝑇

Figure3:Radial heat conduction Apparatus (http//nptel.iitm.ac.in/courses/IITMADRAS/Mechanical_Measurements_Metrology)

Calibration of a Radial flow Thermal Conductivity Apparatus

Figure4:Heat loss calibration data (http//nptel.iitm.ac.in/courses/IITMADRAS/Mechanical_Measurements_Metrology)

Transient Hot Wire Method • Based on detection of the temporal temperature rise in a thin wire immersed in the test material

• S=slope of T(r , t) vs ln (t) curve

Figure5:Transient hot wire method [4]

References [1]

Experimental Methods For Engineers,J.P.Holman, McGraw-Hill Series in Mechanical Engineering,8th Edition,2012

[2]

E.R.G. Eckert and R.J. Goldstein: Measurements in Heat Transfer, McGraw Hill, 1976

[3]

http://nptel.iitm.ac.in/courses/IITMADRAS/Mechanical_Measurements_

[4]

Experimental Set-Up for The Measurement of The Thermal Conductivity of Liquids, Codreanu, V. N. Obreja, National Institute for Research and Development in Micro technologies (IMTBucharest), Romania. Romanian Journal Of Information Science And Technology Volume 10, Number 3, 2007, 215-231

Thank you,,,,,,