Sometimes we can develop equations and solve practical problems by knowing nothing more than the dimensions of the key parameters in the problem. For example, consider the heal Joss through a window in a building. Window efficiency is rated in terms of "R value," which has units of (ft2 · h · °F)/Btu. A certain manufacturer advertises a double-pane window with an R value of 2,5. The same company produces a. triple-pane window with an R value of 3.4. In either case the window dimensions are 3 ft by 5 ft. On a given winter day, the temperature difference between the inside and outside of the building is 45°F. (a) Develop an equation for the amount of heat lost in a given time period
(b) How much heat (in Btu) is lost through the triple-pane window in one 24-h period?
(c) Suppose the building is heated with propane gas, which costs $3.25 per gallon. The propane burner is HO percent efficient. Propane has approximately 90,000 Btu of available energy per gallon. In that same 24-h period, how much money would a homeowner save per window by installing triple-pane rather than double-pane windows?
(d) Finally, suppose the homeowner buys 20 such triple-pane windows for the house. A typical winter has the equivalent of about 120 heating days at a temperature difference of 45°F. Each triple-pane window costs $85 more than the double-pane window. Ignoring interest and inflation, how many years will it take the homeowner to make up the additional cost of the triple-pane windows from heating bill savings?
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Fluid Mechanics
- Return to Required information Sometimes equations can be developed and practical problems solved by knowing nothing more than the dimensions of the key parameters. For example, consider the heat loss through a window in a building. Window efficiency is rated in terms of R value, which has the unit of ft2-hr-F/Btu. A certain manufacturer offers a double-pane window with R=2.5 and also a triple-pane window with R=3.4. Both windows are 3.5 ft by 5 ft. On a given winter day, the temperature difference between the inside and outside is 45°F. Assume that a homeowner buys 20 such triple-pane windows for the house. A typical winter equals about 120 heating days at AT = 45°F. Each triple-pane window costs $85 more than a double-pane window. Ignoring interest and inflation, how many years will it take the homeowner to make up the additional cost of the triple-pane windows from heating bill savings? 9.5 years S MacBook Air !arrow_forwardAs we explained in earlier chapters, the air resistance to the motion of a vehicle is something important that engineers investigate. The drag force acting on a car is determined experimentally by placing the car in a wind tunnel. The air speed inside the tunnel is changed, and the drag force acting on the car is measured. For a given car, the experimental data generally is represented by a single coefficient that is called drag coefficient. It is defined by the following relationship: F, where air resistance for a car that has a listed C, = drag coefficient (unitless) measured drag force (N) drag coefficient of 0.4 and width of 190 cm and height of 145 cm. Vary the air speed in the range of 15 m/sarrow_forwardCardiologists use the short-range scaling exponent α1, which measures the randomness of heart rate patterns, as a tool to assess risk of heart attack. The article “Applying Fractal Analysis to Short Sets of Heart Rate Variability Data” compared values of α1 computed from long series of measurements (approximately 40,000 heartbeats) with those estimated from the first 300 beats to determine how well the long-term measurement (y) could be predicted the short-term one (x). Following are the data (obtained by digitizing a graph). Short Long 0.54 0.55 1.02 0.79 1.4 0.81 0.88 0.9 1.68 1.05 1.16 1.05 0.82 1.05 0.93 1.07 1.26 1.1 1.18 1.19 0.81 1.19 0.81 1.2 1.28 1.23 1.18 1.23 0.71 1.24 Note: This problem has a reduced data set for ease of performing the calculations required. This differs from the data set given for this problem in the text. Compute the least-squares line for predicting the long-term measurement from the short-term measurement.…arrow_forwardSurface tension is due to uneven molecular attraction between immiscible fluid media and solid surface. Its importance is governed by dimensionless parameter Which of the Inertia force called Weber number given by We = Surface tension force™ following statements are correct? (i) Surface tension becomes important when the length scale L is small such as in ink jet printing, coating of thin film, oxygenation of water by falling rain drops. (ii) 111 Super-hydrophobicity can be achieved by micro/nano texturing of surface such as in lotus leave, butterfly and dragonfly wing. During oil spill in ocean, oil floats and spreads out due to gravity. By conservation of mass, the oil film becomes thinner as it spreads. Finally, the surface tension force becomes important causing the formation of tiny oil droplets. (A) (B) (C) (D) (i) and (ii) (i) and (iii) (ii) and (iii) All of the abovearrow_forwardYou are developing a porous membrane for use in a dialysis system. The membrane must be able to retain both protein and glucose on the inlet side and allow other, smaller molecules to flow through. You have found that the membrane is 0.25 mm thick and contains long, rectangular pores with a width of 0.1 microns. 57% of the 50 cm^2 membrane surface area is covered with pores. A test fluid (viscosity = 1.5 cP, density = 1015 kg/m^3) is passed through the membrane. You can assume that the test fluid has a composition similar to that of blood plasma. An initial test is run at physiological conditions, and you observe that the flow rate of fluid through the membrane is 500 cm^3/min. Given this data, what must the hydrodynamic pressure drop across the membrane in your test system be in pascals?arrow_forwardThermal conductivity k is a measure of the ability of a material to conduct heat. For conduction heat transfer in the x-direction through a surface normal to the x-direction, Fourier’s law of heat conduction is expressed as: Q=-kA.dT/dx where ?̇ is the rate of heat transfer and A is the area normal to the direction of heat transfer. Determine the primary dimensions of thermal conductivity (k). Look up a value of k and verify that its SI units are consistent with your result. Write a set of primary SI units for k.arrow_forwardYou are the mechanical engineer supervising the layout of a piping system. In a certain portion of the pipe, the specifications are as follows: length of pipe is 10m, inside diameter of 30cm, outside diameter of 30.5cm, maximum allowable speed of 15m/s and a coefficient of 0.003456. If the uncertainties are 0.02mm for length, 0.8mm for the diameters and 0.1mm/s for the velocity, what loss of head will be imminent in this pipe? In the piping system above, what is the uncertainty in computed head loss contributed by the velocity of the pipe?arrow_forwardYou are the mechanical engineer supervising the layout of a piping system. In a certain portion of the pipe, the specifications are as follows: length of pipe is 10m, inside diameter of 30cm, outside diameter of 30.5cm, maximum allowable speed of 15m/s and a coefficient of 0.003456. If the uncertainties are 0.02mm for length, 0.8mm for the diameters and 0.1mm/s for the velocity, what loss of head will be imminent in this pipe?arrow_forwardYou are the mechanical engineer supervising the layout of a piping system. In a certain portion of the pipe, the specifications are as follows: length of pipe is 10m, inside diameter of 30cm, outside diameter of 30.5cm, maximum allowable speed of 15m/s and a coefficient of 0.003456. If the uncertainties are 0.02mm for length, 0.8mm for the diameters and 0.1mm/s for the velocity, what loss of head will be imminent in this pipe? With all the above uncertainties, what is the total uncertainty in the head loss?arrow_forwardQl: The viscosity in industrial measurement continue to use the CGS system of Lunits, since centimeters and grams vield convenient numbers for many fluids. The absolute viscosity () unit is the poise, I poise = 1 gtem. s). The kinematic viscosity (v) unit is the stohes, I stokes = 1 em /s. Water at 20C has u = 001 poise and also V= 0.01 stokes. Express these resalts in (a) SI and (h) BG tanits.arrow_forwardIn the following section, at least 2 to up to 5 answers may be correct. 1) For a fluid, the assumption (simplifying notion) of incompressibility has important consequences: Pascal’s principle: a change of pressure in an enclosed fluid at rest is transmitted undiminished to all points in the fluid. pressure changes are transmitted immediately from one place to another. the speed of sound then is infinite (just within this approximation). pressure becomes unpredictable. none of the above. 2) Archimedes’ principle can be summarized as: an immersed object is buoyed up by a force equal to the weight of the fluid it displaces. a bathtub is fun, and may lead to important physical discoveries regarding the volume of an object and how much water it displaces, and the weight of that amount of water. boats swim because of the work done by sailors. submarines are always doomed. fish swim because they are less heavy than water 3) A…arrow_forwardLook up the metric specifications for a car of your choice (body, trunk, engine sizes, and gas consumption) and a home appliance such as an air-conditioning unit (size, cooling capacity, and energy consumption). Convert your findings to U.S. Customary units.arrow_forwardarrow_back_iosSEE MORE QUESTIONSarrow_forward_ios
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