Ahot-water stream at 70°c enters an adiabatic mixing chamber with a mass flow rate of 3.6 kg/s, where it is mixed with a stream of cold water at 20°c. if the mixture leaves the chamber at 42°c, determine
(a) the mass flow rate of the cold water and
(b) the rate of entropy generation during this adiabatic mixing process. assume all the streams are at a pressure of 200 kpa

a) 4.58 kg/s

b) 0.105 kW/K

Explanation:

a) For the energy balance

qin = qout

min*cin*Tin = mout*cout*Tout

Where min is the mass flow that is entering the system, cin is the specific heat of the substance that is entering, Tin is the temperature of the substance that is entering, and the out is from the substances that are leaving the system. Because there is only water in the system cin = cout. The energy balance will be then:

mh*Th + mc*Tc = (mh + mc)*Tmix

Where h is from the hot water, c is from the cold water, and mix for the mixture that leaves the chamber. Th = 70ºC + 273 = 343 K, Tc = 20ºC + 273 = 293 K, Tmix = 42ºC + 273 = 315 K

3.6*343 + mc*293 = (3.6 + mc)*315

293mc - 315mc = 3.6*315 - 3.6*343

- mc*(315 - 293) = -3.6*(343 - 315)

mc = 3.6*(343 - 315)/(315 - 293)

mc = 3.6*28/22

mc = 4.58 kg/s

b) The entropy balance is:

Sin + Sout - Sgen = ΔSsystem

Where Sin is the entropy of the entering substances, Sout the entropy that the leaving substances, and Sgen the entropy that is generated in the process.

The entropy variantion for an adiabatic process is 0, so ΔSsystem = 0

Sgen = Sin + Sout

The sum of entropy is:

m*cp*ln(Tfinal/Tinitial), where cp is the specific heat (4.184 kJ/kg.K)

Sgen = mc*cp*ln(Tmix/Tc) + mh*cp*ln(Tmix/th)

Sgen = 4.58*4.184*ln(315/293) + 3.6*4.184*ln(315/343)

Sgen = 1.3874 + (-1.2827)

Sgen = 0.105 kW/K

in 1783 cavendish was working with his hydrogen gas and found that when he burned it and condensed the vapor it was water. this was very significant since it disproved the greek theory of elements completely since water was not a simple substance but the product of two gases. it was lavoisier who hearing of the experiment gave cavendish's gas the name hydrogen ("water-producer") and added that hydrogen burned by combining with oxygen and that water was a hydrogen-oxygen combination.

explanation:

pre-sliced bread is a relatively recent development in baking technology. previous to that, it was always sold in a bakery in daily-made contiguous loaves with no wrapping, and was moderately perishable, usually being used with a day or two (which is why every bakery had a discounted “day-old” bread table.) therefore, pre-sliced bread would grow stale quickly. the hard crust provided a protective layer, and once the exposed dry slice left from the previous cut was removed, the interior was still soft.

what keeps bread soft is the moisture retained in the crumb (the light spongy part that’s not crust). the reason the crust develops is the surface of the dough gets exposed to hot dry air during baking which browns the outer layer and seals it, preventing all the moisture from escaping the dough. longer exposure to air will also eventually cause your bread to lose flavor and become stale.

people were used to cutting slices as they needed them and storing bread in a specially-made wooden or metal breadbox or pail with a lid to preserve it longer, but the first automatically sliced commercial loaves were produced on july 6, 1928, in chillicothe, missouri, using a machine invented by otto rohwedder, an iowa-born, missouri-based jeweler.[1] pre-sliced bread would indeed dry out, were it not for another 20th century technology: the plastic bag. bread could now be pre-sliced, and if the enclosing bag remains sealed as much as possible, the bread will remain soft and fresh for longer than if sitting unwrapped on a shelf or enclosed in a bread box that is not air-tight. sealed packaging meant bread could also be sold on store shelves and eliminate frequent trips to a bakery.

sealing bread in a plastic bag had another effect: the moisture in the loaf will re-distribute into the crust and soften it, which is sometimes more pleasing in a sandwich - especially for children whose teeth might not yet be strong enough to tear through a hard crust. a homemade loaf of bread can be sealed in a bag overnight and the crust will become soft like store-bought sandwich bread.

these were novel changes in how bread was produced and sold, which led to the coining of the phrase “the greatest thing since sliced bread.”