Chemistry Question

Science

ashford university

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See the attached study notes. Should not be tuff..................................

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EML 4142 Exam 1 Structure Time Limit: 70 minutes (The exam will be open for 85 minutes. The extra time is for students to upload work for Problems 5 and 6). Tentative structure and tentative point distribution: Question 1: (5 points) multiple-choice, regarding concepts or brief calculation in Chapter 1 Question 2: (5 points) multiple-choice, regarding concepts or brief calculation in Chapter 1 Question 3: (5 points) multiple-choice, regarding concepts or brief calculation in Chapter 1 Question 4: (5 points) multiple-choice, regarding concepts or brief calculation in Chapter 1 Question 5: (40 points) problem-solving question from Chapter 2, file upload is required Question 6: (40 points) problem-solving question from Chapter 2, file upload is required • Click this link Optional: Learn how to upload your work for Exam 1: EML4142-21Spring 0V01 (ucf.edu) to practice how to upload your work. • The exam window will close at 1:25 sharp. No extension will be provided. Please make sure you know how to scan your work and upload your files onto the system prior to the exam. EML4142 Exam 1 Coverage Modules Lecture Slides Sections Homework Assignment 1&2 (video available) Chapter 1 1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9 HW #1 3 (video available) Chapter 2 1-5, 1-6, 1-7, 1-8, 1-9 HW #2 Eml4142 Heat Transfer Module 1&2 Chapter 1 Introduction and Basic Concepts Practice Questions Instructor: Tian Tian Practice Question 1 You’ve experienced convection cooling if you’ve ever extended your hand out the window of a moving vehicle or into a flowing water stream. With the surface of your hand at a temperature of 30 ℃, determine the convection heat flux for (a) a vehicle speed of 35 km/h in air at -5 ℃ with a convection coefficient of 40 W/𝑚2 ∙ 𝐾 (b) a velocity of 0.2 m/s in a water stream at 10 ℃ with a convection coefficient of 900 W/𝑚2 ∙ 𝐾. Practice Question 2 Why do we feel chilled in the winter but comfortable in the summer under conditions for which the same room temperature is maintained by a heating or cooling system? Practice Question 3 One way of measuring the thermal conductivity of a material is to sandwich an electric thermofoil heater between two identical rectangular samples of the material and to heavily insulate the four outer edges, as shown in the figure. Thermocouples attached to the inner and outer surfaces of the samples record the temperatures. During an experiment, two 0.5-cm thick samples 10 cm × 10 cm in size are used. When steady operation is reached, the heater is observed to draw 25 W of electric power, and the temperature of each sample is observed to drop from 82°C at the inner surface to 74°C at the outer surface. Determine the thermal conductivity of the material at the average temperature. Practice Question 4 An 800-W iron is left on the iron board with its base exposed to the air at 20℃. The convection heat transfer coefficient between the base surface and the surrounding air is 35 𝑊/𝑚2 ∙ 𝐾. If the base has an emissivity of 0.6 and a surface area of 0.02 𝑚2 , determine the temperature of the base of the iron. Created by Dr. Tian. Do NOT post on any websites. Eml 4142 Heat Transfer Module 3 Chapter 2 Heat Conduction Equation Practice Questions Instructor: Tian Tian Practice Question 2.1 Created by Dr. Tian. Do NOT post on any websites. Assume steady-state, one-dimensional heat conduction through the axisymmetric shape. Assuming constant properties and no internal heat generation, sketch the temperature distribution? 2 Practice Question 2.2 Created by Dr. Tian. Do NOT post on any websites. A long copper bar with width w much greater than its thickness L, initially in thermal equilibrium with a heat sink, then suddenly heated by passage of an electric current 𝑒𝑔𝑒𝑛 ሶ 𝑒𝑔𝑒𝑛 ሶ 1) The differential equation? 2) The initial condition? 3) The boundary condition? (A) the 1st B. C. at x=0, the 1st B. C. at x=L (B) the 1st B. C. at x=0, the 2nd B. C. at x=L (C) the 1st B. C. at x=0, the 3rd B. C. at x=L (D) the 2nd B. C. at x=0, the 2nd B. C. at x=L (E) the 2nd B. C. at x=0, the 3rd B. C. at x=L 3 Practice Question 2.3 Created by Dr. Tian. Do NOT post on any websites. (Similar to Problem 2-59 in the textbook) Consider the base plate of an 800-W household iron with a thickness of 𝐿 = 0.6 𝑐𝑚, base area of 𝐴 = 160 𝑐𝑚2 , and thermal conductivity of 𝑘 = 60 𝑊/𝑚 ∙ 𝐾. The inner surface of the base plate is subjected to uniform heat flux generated by the resistance heaters inside. When steady operating conditions are reached, the outer surface temperature of the plate is measured to be 112℃. Disregarding any heat loss through the upper part of the iron, evaluate the inner surface temperature. 4 Created by Dr. Tian. Do NOT post on any websites. 5 Practice Question 2.4 Created by Dr. Tian. Do NOT post on any websites. (Problem 2-74 in the textbook) Liquid ethanol is a flammable fluid that has a flashpoint at 16.6 ℃. At temperatures above the flashpoint, ethanol can release vapors that form explosive mixtures, which could ignite when source of ignition is present. In a chemical plant, liquid ethanol is being transported in a pipe (𝑘 = 15 𝑊/𝑚 ∙ 𝐾) with an inside diameter of 3 cm and a wall thickness of 3 mm. The pipe passes through areas where occasional presence of ignition source can occur, and the pipe’s outer surface is subjected to a heat flux of 1 𝑘𝑊/𝑚2 . The ethanol flowing in the pipe has an average temperature of 10℃ with an average convection heat transfer coefficient of 50 𝑊/𝑚2 ∙ 𝐾. Your task as an engineer is to ensure that the ethanol is transported safely and prevent fire hazard. Determine the variation of temperature in the pipe wall and the temperatures of the inner and outer surfaces of the pipe. Are both surface temperatures safely below the flashpoint of liquid ethanol? See Video "EML4142 Module 3 Chapter 2 Practice Question 2.4" for detailed explanation https://www.youtube.com/watch?v=8XcpiA5z7fw&feature=youtu.be 6 Created by Dr. Tian. Do NOT post on any websites. 7 Extra Practice Question 1 KNOWN: Length, diameter, surface temperature and emissivity of steam line. Temperature and convection coefficient associated with ambient air. Efficiency and fuel cost for gas fired furnace. FIND: (a) Rate of heat loss, (b) Annual cost of heat loss. SCHEMATIC: = 0.8 ASSUMPTIONS: (1) Steam line operates continuously throughout year, (2) Net radiation transfer is between small surface (steam line) and large enclosure (plant walls). ANALYSIS: (a) From Eqs. (1.3a) and (1.7), the heat loss is ( ) 4 ⎤ q = q conv + q rad = A ⎡ h ( Ts − T∞ ) + εσ Ts4 − Tsur ⎣ ⎦ where A = π DL = π ( 0.1m × 25m ) = 7.85m 2 . Hence, ( ) q = 7.85m 2 ⎡10 W/m 2 ⋅ K (150 − 25 ) K + 0.8 × 5.67 × 10−8 W/m 2 ⋅ K 4 4234 − 2984 K 4 ⎤ ⎣ ⎦ q = 7.85m 2 (1, 250 + 1,095 ) W/m 2 = ( 9813 + 8592 ) W = 18, 405 W < (b) The annual energy loss is E = qt = 18, 405 W × 3600 s/h × 24h/d × 365 d/y = 5.80 × 1011 J With a furnace energy consumption of E f = E/ηf = 6.45 × 1011 J, the annual cost of the loss is C = Cg Ef = 0.02 $/MJ × 6.45 × 105 MJ = $12,900 < COMMENTS: The heat loss and related costs are unacceptable and should be reduced by insulating the steam line. Extra Practice Question 2 KNOWN: Steady-state, one-dimensional heat conduction through an axisymmetric shape. FIND: Sketch temperature distribution and explain shape of curve. SCHEMATIC: ASSUMPTIONS: (1) Steady-state, one-dimensional conduction, (2) Constant properties, (3) No internal heat generation. & in − E& out = 0, it ANALYSIS: Performing an energy balance on the object according to Eq. 1.12c, E follows that E& in − E& out = q x bg and that q x ≠ q x x . That is, the heat rate within the object is everywhere constant. From Fourier’s law, q x = − kA x dT , dx and since qx and k are both constants, it follows that Ax dT = Constant. dx That is, the product of the cross-sectional area normal to the heat rate and temperature gradient remains a constant and independent of distance x. It follows that since Ax increases with x, then dT/dx must decrease with increasing x. Hence, the temperature distribution appears as shown above. COMMENTS: (1) Be sure to recognize that dT/dx is the slope of the temperature distribution. (2) What would the distribution be when T2 > T1? (3) How does the heat flux, q ′′x , vary with distance? Extra Practice Question 3 2-24 2-61 An engine housing (plane wall) is subjected to a uniform heat flux on the inner surface, while the outer surface is subjected to convection heat transfer. The variation of temperature in the engine housing and the temperatures of the inner and outer surfaces are to be determined for steady one-dimensional heat transfer. Assumptions 1 Heat conduction is steady and one-dimensional. 2 Thermal conductivity is constant. 3 There is no heat generation in the engine housing (plane wall). 4 The inner surface at x = 0 is subjected to uniform heat flux while the outer surface at x = L is subjected to convection. Properties Thermal conductivity is given to be k = 13.5 W/m∙K. Analysis Taking the direction normal to the surface of the wall to be the x direction with x = 0 at the inner surface, the mathematical formulation can be expressed as d 2T 0 dx 2 Integrating the differential equation twice with respect to x yields dT  C1 dx T ( x)  C1 x  C2 where C1 and C2 are arbitrary constants. Applying the boundary conditions give q0 k x  0: k dT (0)  q 0  kC1 dx x  L: k dT ( L)  h[T ( L)  T ]  h(C1 L  C 2  T ) dx  C1    kC1  h(C1 L  C2  T ) Solving for C2 gives q  k k   C2  C1   L   T  0   L   T h k h     Substituting C1 and C2 into the general solution, the variation of temperature is determined to be k  T ( x)  C1 x  C1   L   T h   T ( x)  q 0  k    L  x   T k h  The temperature at x = 0 (the inner surface) is T (0)  q 0  k 6000 W/m2 13.5 W/m  K    0.010 m  35C  339C   L   T   2 k h 13.5 W/m  K  20 W/m  K   The temperature at x = L = 0.01 m (the outer surface) is T ( L)  q 0 6000 W/m2  T   35C  335C h 20 W/m2  K Discussion The outer surface temperature of the engine is 135°C higher than the safe temperature of 200°C. The outer surface of the engine should be covered with protective insulation to prevent fire hazard in the event of oil leakage. PROPRIETARY MATERIAL. © 2015 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course preparation. If you are a student using this Manual, you are using it without permission. Extra Practice Question 4 ...
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