Concept explainers
What is the physical significance of the Biot number? Is the Biot number more likely to be larger for highly conducting solids or poorly conducting ones?
To discuss: The Physical significance of Biot number and define whether its value is larger for highly conducting solids or poorly conducting solids.
Explanation of Solution
The Biot number is defined as the ratio of internal conduction resistance to the external convection resistance of the body.
The Biot number is used for the analysis of lumped heat capacitive systems, in which thermal internal conduction resistance is much lesser than the external convection resistance. And the condition for its applicability is Bi ? 0.1.
From the formula of Biot number, it can be defined that its value is inversely proportional to the thermal conductivity of the body.
Thus, with a larger value of k, the Biot number will be lesser. Hence, due to lower thermal conductivity, the value of the Biot number will be larger for poorly conducting solids. Also, the poorly conducting materials possess high conduction resistance, resulting in the larger value of the Biot number.
Want to see more full solutions like this?
Chapter 4 Solutions
Heat and Mass Transfer: Fundamentals and Applications
- A person is found dead at 5 PM in a room whose temperature is 20 °C. The temperature of the body is measured to be 25 °C when found, and the heat transfer coefficient is estimated to be h 8 W/m2 °C. Modeling the body as a 30-cm-diameter, 1.70-m-long cylinder, estimate the time of death of that person. The body can be modeled as a 30 cm diameter, 1.70-m-long cylinder. The thermal properties and body temperature of 37 °C. let The average human body is 72 percent water by mass, and thus we can assume the body to have the properties of water at the average temperature of (37 + 25)/2 = 31 °C; k = 0.617 W/m · °C, p= 996 kg/m³, and Cp=4178 J/kg · °Carrow_forwardhow the mechanisms of heat transfer are applied to the designof structures (materials selection and geometry considerations) for energyconservation and economic purposes. Include illustrations and references.arrow_forwardA glass container has a total wall area (including the lid) of 1.75 m2 and a wall thickness of 1.5 cm. It's filled with 1000 g of ice at 0 C. How many seconds will it take to completely melt all the ice? (Latent heat of fusion is 80 calg and Thermal conductivity of glass is 0.8 W/m K) O 120 s 14, 400 s 2, 800 s 200 sarrow_forward
- Determine the ratio of thermal conductivity for N2 at sea level (T = 300 K, P = 1 atm) versusthe lower stratosphere (T = 230 K, P = 0.25 atm).arrow_forwardSteel rods (1% carbon) that are 1.5 m long and 2 cm in diameter are heated in a furnace to850°C and then quenched in a water bath at 50°C for 40 s as part of a hardening process. Given the convective heattransfer coefficient is 750 W/m2K, find the average temperature of the rods when they are taken out of the bath.Show that lumping the rods is appropriate.arrow_forwardConsider hotdog being cooked in boiling water in a pan. Would the heat transfer be modeled as one-dimensional or two-dimensional? Would the heat transfer be steady or transient? Explain.arrow_forward
- Will definitely upvotearrow_forwardTo warm up milk, it is poured into a thin-walled glass whose dimeter is 6 cm. The height of the milk in the glass is 7 cm. The glass is placed into a large pan filled with hot water at 60o C. The milk is stirred constantly, so that its temperature is always uniform. If the heat transfer coefficient between the water and the glass is 120 W/m2oC, determine how long it will take for the milk to warm up from 3o C to 38o C. Take the properties of the milk to be the same as those of water. Can the milk in this case be treated as a lumped system> Why?arrow_forwardTwo nested spherical tanks with the internal and outer diameters of the 100 cm by 104 cm and 114 cm by 118 cm is used to store hot water at 100 C. Both tanks are made of boron fiber epoxy with different composite compositions. The thermal conductivity of the inner tank is 1.5 W/m K while the outer tank has a thermal conductivity of 0.5 W/m K. The gap between the tanks is filled with air (use properties of air at 50°C). The tank is located in an open environment at 0'C. The outer surface of the tank is white painted and heat transfer between the outer surface of the tank and the surrounding is by natural covection and radiation. The convection heat transfer coefficient at the inner and the outer surface of the pipe is h 20 W/m' K and h 10 W/m K. Determine; a. the rate of heat loss from the tank b. the inside, outside and intermediate surface temperatures. Hint: Take the outer surface temperature as 3°C for radiation calculations.arrow_forward
- The boiling temperature of nitrogen at atmospheric pressure at sea level (1 atm pressure) is -196 °C. Therefore, nitrogen is commonly used in low-temperature scientific studies since the temperature of liquid nitrogen in a tank open to the atmosphere will remain constant at -196 °C until it is depleted. Any heat transfer to the tank will result in the evaporation of some liquid nitrogen, which has a heat of vaporization of 198 kJ/kg and a density of 810 kg/m3 at 1 atm. Consider a 3-m-diameter spherical tank that is initially filled with liquid nitrogen at 1 atm and -196 °C. The tank is exposed to ambient air at 15° C, with a combined convection and radiation heat transfer coefficient of 35 W/m2⋅K. The temperature of the thin-shelled spherical tank is observed to be almost the same as the temperature of the nitrogen inside. Determine the rate of evaporation (in kg/s) of the liquid nitrogen in the tank as a result of the heat transfer from the ambient air if the tank is insulated with…arrow_forwardHEAT TRANSFER SUBJECTarrow_forwarda. What is the heat flux, q"1 [in W/m2], at the left-hand side of layer B? Express your answer as a negative number if the heat flux goes to the left, and as a positive number if the heat flux goes to the right. b.What is the heat flux, q"2 ( in W/m2) at the right-hand side of layer B? Express your answer as a negative number if the heat flux goes to the left, and as a positive number if the heat flux goes to the right. c. What is the temperature, T1, on the left-hand side of layer B, in Celsius? d. What is the temperature, T2, on the right-hand side of layer B, in Celsius?arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning