We measure the flow of granular materials inside a quasi-two-dimensional silo as it drainsand compare the data with some existing models. The particles inside the silo are imagedand tracked with unprecedented resolution in both space and time to obtain their velocityand diffusion properties. The data obtained by varying the orifice width and thehopper angle allow us to thoroughly test models of gravity driven flows inside thesegeometries. All of our measured velocity profiles are smooth and free of the shock-likediscontinuities (‘rupture zones’) predicted by critical state soil mechanics. Onthe other hand, we find that the simple kinematic model accurately capturesthe mean velocity profile near the orifice, although it fails to describe the rapidtransition to plug flow far away from the orifice. The measured diffusion lengthb, the only free parameter in the model, is not constant as usually assumed, but increaseswith both the height above the orifice and the angle of the hopper. We discussimprovements to the model to account for the differences. From our data, we also directlymeasure the diffusion of the particles and find it to be significantly less thanpredicted by the void model, which provides the classical microscopic derivation ofthe kinematic model in terms of diffusing voids in the packing. However, theexperimental data are consistent with the recently proposed spot model, basedon a simple mechanism for cooperative diffusion. Finally, we discuss the flowrate as a function of the orifice width and hopper angles. We find that the flowrate scales with the orifice size to the power of 1.5, consistent with dimensionalanalysis. Interestingly, the flow rate increases when the funnel angle is increased.