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Office Hours changed on Tuesday

Office Hours changed on Tuesday. 5:30 -7:00 in HLS 366 for people who have exam questions. General Question. Test format: What types of questions should we expect? 10 multiple choice questions (5 points each) Short problems (similar to previous prelims)

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Office Hours changed on Tuesday

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  1. Office Hours changed on Tuesday • 5:30 -7:00 in HLS 366 for people who have exam questions

  2. General Question • Test format: What types of questions should we expect? • 10 multiple choice questions (5 points each) • Short problems (similar to previous prelims) • How far should we be able to extrapolate beyond what is in the notes? • You should understand the fundamental concepts and be able to talk about those concepts with your friends • Algebra and arithmetic are expected

  3. Velocity Shear (wall on fluid) What are examples of major and minor losses? • Major: Caused byshear with the solid surface • Pipe walls • Flocculator baffle surfaces (insignificant) • Minor: Flow expansions (analogous to pressure drag) • Orifice, elbow, valve, any place where streamlines are diverging!

  4. Origin of • How is this equation used for rapid mix? • How is this equation used for flocculator design? • Draw a picture. What are the parameters in each case?

  5. Orifice Diameter to Obtain Target Mixing (Energy Dissipation Rate, Kolmogorov Length Scale) Substitute for DJet and solve for DOrifice The orifice must be smaller than this to achieve the target energy dissipation rate

  6. How does aggregation occur? • Coat flocs with nanoglobs of coagulant precipitate to make them sticky, • then provide energy to create velocity gradients • Collisions!!!!

  7. Reflection Questions • What are some alternate geometries? • How else could you generate head loss to create flocs? 0.4S 0.6S

  8. Reflection Questions • Why can’t we use the relationto calculate the residence time in a flocculator and then use that to calculate the number of baffles? • What is the relationship between potential energy loss and the average energy dissipation rate in a flocculator? • How did AguaClara get around the 45 cm limitation?

  9. Two changes to for nonuniformity • We design based on • For max efficiency flocculator, 600s and 10 mW/kg max, what is y?

  10. Flocculation Efficiency isthe flocculation efficiency given the non uniform energy dissipation rate CFD results 2 3 4 5 6 7 8

  11. Variability of energy dissipation rate e Is more uniform when H/S is between 3 and 5. ae relation between the maximum and average energy dissipation rate

  12. Transition Flow Ratewhere H/S=5 ~5 Set the 2 equations for S equal For 10 mW/kg, H=2m, W=0.6 m, then Q=42 L/s

  13. Design steps:Given H,W, Q, eMax, and y For H/S>5 For H/S<5 • Calculate S • Calculate yB • Calculate the number of spaces and head loss

  14. Energy Dissipation Rate ɛ • Why is ɛ important in the flocculator? • Provides the energy for chaotic eddies that move flocs at different velocities • How did AguaClara choose ɛmax= 10mW/kg? • Lower energy dissipation rate causes flocs to settle before they get to the sed tank • Higher e will require more head loss • Optimal value is not yet known

  15. What controls ɛ in a flocculator flocculator? • What is ɛ controlled by? • What change in geometry is required to change ɛ? • Flocculator design! • What controls collision potential? • What controls energy dissipation rate? Energy dissipation rate, residence time, number of baffles Baffle spacing

  16. Floc size • Will the size of flocs increase or decrease if the flow rate in the flocculator decreases? • Why with the decrease of ɛ, dothe flocsgrow bigger? Flocs can grow to a larger size because the shear on the surface of the floc will be lower because the velocity gradients are lower.

  17. Flocculator Geometry • Why must the number of baffles be odd for first (n-1) channels but even for the last channel? • Why is the spacing at the bottom of baffles 1.5S? (tradition) • Why is there a height change from the beginning and the end of flocculator? • Energy grade lines and hydraulic grade lines

  18. Flocculator Geometry • What are the geometry design constraints for the flocculator? • If we have a small fixed space to construct, we are suppose to create a tall plant even knowing that the H/S will be high. How do we know the right H for this case? • We use the depth of the sedimentation tank to set the depth of the flocculator • Otherwise you could experiment (cost analysis)

  19. Flocculator Efficiency • Can you clarify the H/S ratio limit? • Why does a transition happen at H/S=5? • Transition marks the geometry where the jet has the opportunity to fully expand before going around the next bend H/S = 4 Energy dissipation zone limited by H Energy dissipation zone limited by S H/S = 10

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