Engineering, 01.10.2019 05:30 Itslegit3

Ahollow aluminum sphere, with an electrical heater in the center, is used in tests to determine the thermal conductivity of insulating materials. the inner and outer radii of the sphere are 0.15 and 0.18 m, respectively, and testing is done under steady-state conditions with the inner surface of the aluminum maintained at 250°c. in a particular test, a spherical shell of insulation is cast on the outer surface of the sphere to a thickness of 0.12 m. the system is in a room for which the air temperature is 20°c and the convection coefficient at the outer surface of the insulation is 30 w/m2·k. if 70w are dissipated by the heater under steady-state conditions, what is the thermal conductivity of the insulation?

Answers: 3

Show answers

Answers

Answer from: burritomadness

$k_{ins} = 0.0541 W/m. K$

Explanation:

Given data:

inner radii of sphere is 0.15

outer radii of sphere is 0.18 m

thickness of given insulation is 0.12m

outer radius of given insulation is 0.18 + 0.12 = 0.30 m

inner temperature of surface os 250 degree C = 523 k

$T_{\infity} = 293 K$

convective coffcient is h= 30 W/m^2 -K

For given condition, rate of heat transfer is given as

$q = \frac{T_s -T_{\infty}}{R}$

where R is thermal resistance

$R = R_{cond} + R_{cond, ins} + R_{conv}$

$R =\frac{1}{4\pi K_a} [\frac{1}{r_i} - \frac{1}{r_o}] + \frac{1}{4\pi K_{ins}} [\frac{1}{r_0} - \frac{1}{r}] + \frac{1}{4\pi r^2 h}$

fro aluminum at T = 523 K, Thermal conductivity is K_a = 230 W/m. K

$R =\frac{1}{4\pi \times 230} [\frac{1}{0.15} - \frac{1}{0.18}] + \frac{1}{4\pi K_{ins}} [\frac{1}{.18} - \frac{1}{0.30}] + \frac{1}{4\pi 0.30^2 30}$

$R = 0.02985 + \frac{0.1763}{k_{ins}} K/W$

FROM

$q = \frac{T_s -T_{\infty}}{R}$

$70 = \frac{523 - 293}{0.02985 + \frac{0.1763}{k_{ins}}}$

solving for $R_{ins}$

$0.02985 + \frac{0.1763}{k_{ins}} = 3.285$

$\frac{0.1763}{k_{ins}} = 3.255$

$k_{ins} = 0.0541 W/m. K$

Answer from: Quest

answer:

200k

explanation:

Answer from: Quest

Infrared sensor is most used sensor in wireless technology where remote controlling functions and detection of surrounding objects/ obstacles are involved. this post will discuss about what is infrared sensor, its working principle, how it works, types, applications, advantages and disadvantages. what is infrared sensor

ir sensor is a simple electronic device which emits and detects ir radiation in order to find out certain objects/obstacles in its range. some of its features are heat and motion sensing.

ir sensors use infrared radiation of wavelength between 0.75 to 1000µm which falls between visible and microwave regions of electromagnetic spectrum. ir region is not visible to human eyes. infrared spectrum is categorized into three regions based on its wavelength i.e. near infrared, mid infrared, far infrared.

wavelength regions of infrared spectrum

near ir – 0.75µm to 3 µm
mid ir – 3 µm to 6 µm
far ir – > 6 µm
1 introduction to infrared sensor

fig. 1 – introduction to infrared sensor

working principle of infrared sensor

infrared sensors works on three fundamental physics laws:

planck’s radiation law: any object whose temperature is not equal to absolute zero (0 kelvin) emits radiation.
stephan boltzmann law: the total energy emitted at all wavelengths by a black body is related to the absolute temperature.
wein’s displacement law: objects of different temperature emit spectra that peak at different wavelengths that is inversely proportional to temperature.
key elements of infrared detection system

the key elements of infrared detection system are:

ir transmitter

ir transmitter acts as source for ir radiation. according to plank’s radiation law, every object is a source of ir radiation at temp t above 0 kelvin. in most cases black body radiators, tungsten lamps, silicon carbide, infrared lasers, leds of infrared wavelength are used as sources.

transmission medium

as the name suggests, transmission medium provides passage for the radiation to reach from ir transmitter to ir receiver. vacuum, atmosphere and optical fibers are used as medium.

ir receiver

generally ir receivers are photo diode and photo transistors. they are capable of detecting infrared radiation. hence ir receiver is also called as ir detector. variety of receivers are available based on wavelength, voltage and package.

ir transmitter and receivers are selected with matching parameters. some of deciding specifications of receivers are photosensitivity or responsivity, noise equivalent power and detectivity.

Another question on Engineering

Engineering, 04.07.2019 18:20

Ahe-xe mixture containing a 0.75 mole fraction of helium is used for cooling electronics in an avionics application. at a temperature of 300 k and atmospheric pressure, calculate the mass fraction of helium and the mass density, molar concentration and molecular weight of the mixture. if the cooling capacity is 10 l, what is the mass of the coolant?

Answers: 3

Answer

Engineering, 04.07.2019 18:20

Air flows over a heated plate àt a velocity of 50m/s. the local skin factor coefficient at a point on a plate is 0.004. estimate the local heat transfer coefficient at this point.the following property data for air are given: density = 0.88kg/m3 , viscosity 2.286 x 10 ^-5 kgm/s , k = 0.035w/mk ,cp = 1.001kj/kgk. use colburn reynolds analogy.

Answers: 1

Answer

Engineering, 04.07.2019 19:10

An engine, weighing 3000 n, is supported on a pedestal mount. it has been observed that the engine induces vibration into the surrounding area through its pedestal at the maximum operating speed. determine the stiffness of the dynamic vibration absorber spring in (n/m) that will reduce the vibration when mounted on the pedestal. the magnitude of the exciting force is 250 n, and the amplitude of motion of the auxiliary mass is to be limited to 2 mm note: in this question type-in right numbers, no decimals, no fractions, no unit. approximate to right number if needed

Answers: 3

Answer

Engineering, 04.07.2019 19:10

Agas is compressed from vi 0.3 m, p 1 bar to of v2 0.1 m3, p2--3 bar. pressure and volume are related linearly during the process. for the gas, find the work, in kj.

Answers: 2

Answer

You know the right answer?

Ahollow aluminum sphere, with an electrical heater in the center, is used in tests to determine the...

Questions