Why does the pressure of fluid increase when the diameter of the pipe increases? Assuming mass flow rate doesn't change and it's a closed system. Why does going from a smaller pipe to a large pipe cause an increase in pressure? What is the kinetic energy converted to as the diameter gets larger?
 A: The answer to this is highly dependent upon on the conditions (constraints) that the fluid is under (i.e. compressible flow, sonic, supersonic, etc.). I will answer it in the most basic set of conditions, and you can remove some of the constraints to see for yourself how it affects the fluid properties.
If we assume that the fluid is of constant density (incompressible), then the statement of continuity requires that (assuming steady state) the velocity decreases inversely proportional to the area increase. In mathematical terms, if we name subscript "1" as the position where the area is $A_1$ and subscript "2" as the position where the area is $A_2$, then the velocity at position 2 is
$$ V_2 = V_1\frac{A_1}{A_2} $$
The above equation is purely a kinematic condition that is taken from the statement of mass conservation for constant density, steady state flows. Next, if we take the further assumption that the fluid is inviscid and adiabatic (isentropic), then we know that the total pressure must be conserved. Thus,
$$ P_0 = P + \frac{1}{2}V^2 = constant $$
So, if our velocity decreases due to an area increase, then it must be that our static pressure must increase in order to conserve the total pressure. This result can also be derived more generally by using Bernoulli's principle where the potential energy of the fluid is also accounted for.
Remember that this only holds in the limited regime where the above assumptions are true. 
A: Part of the answer is that, if the pipe diameter is getting larger, the parcels of fluid traveling through the pipe have to be decelerating.  So the downstream force pushing backwards (downstream pressure times area) has to be greater than the upstream force pushing forwards (upstream pressure times area).  This doesn't tell the whole story, however, because there is also forward force applied to the fluid by the diverging wall.  But, when all this is taken into account, the upstream pressure still has to be greater than the downstream pressure.  So basically, the mechanistic cause of all this Newtons 2nd law, $F_{net}=ma$.
